Because somebody asked again over at FFR and because LJ wouldn't let my long-winded ass post it in comments, I'm posting this Too Long thing here.
Note: This has become something of a work in progress. Links and additional information are still being added as I think of them or as things are suggested to me in the comments. Any new additions after this will be tacked on as appendixes to the end of this post. (Actually, not entirely true, in most cases the references have been worked into the original text of the article.)
Updated 09/18/07
Note to Non-Transformers Fans: "Transformers" is a catch-all term for characters and media produced by Hasbro. The first Transformers toys came out in 1984 and over the last 23 years, there have been numerous and often conflicting variations of Transformers from the old-school, Generation 1 characters most commonly associated with the name to Beast Wars/Beast Machines, Robots in Disguise, Armada/Energon/Cybertron and the current live-action Transformers movie. There are versions of Transformers -- toys, comics and cartoons -- that were released only in Japan and comic versions that are exclusive to the UK. Essentially, Transformers is a buffet and fans commonly pick and choose only those aspects that appeal to them. You Harry Potter fans think you have it rough with movie vs. book? You ain't seen nothing yet.
The point of this essay isn’t to convince people that Transformers must have sex or even that Transformer sex is officially canonical; the point of this essay is to show that those fic writers who do write Transformers in adult situations aren’t pulling their ideas out of nowhere. Transformers having romantic relationships and/or sex is a fairly reasonable extrapolation from canon for several series.
Table of Contents:
I. Introduction
II. Gender
III. Romance
A. Interspecies Romance
B. Slash
C. Asexuality
IV. Transformers and Reproduction
V. The Nitty-Gritty: How Transformers Have Sex.
A. Stickyfic
B. Plug&Play
Addendum I -- Plug&Play Methods
-----------
I. Introduction:
Ok, I've written about this before but I can't find the thread it was in, so here we go again (note: this is very long winded because I'm trying to get this all out, save it and just post it as needed the next time someone asks; apologies for the Teal Deer):
The first thing to keep in mind is that the Transformers, canonically, are sentient robots. Unlike many other fictional robots, they are capable of thinking independently and experience an emotional range very similar to humans -- anger, hate, love, jealousy, desire, etc.
Non-fans/new fans may want to take a look at the Transformers wiki's article on Transformers for more information about Transformers as a species.
II. Gender:
In addition to being human-like in terms of intelligence and emotional range, Transformers are also, in many cases gendered beings since there are both male and female Transformers depicted in several different continuities (though in many cases, females are rare for reasons that have never been explored in canon, but which may have something to do with Hasbro's belief that female characters don't sell as well to the Transformers' target audience, little boys).
In the G1 comics, female Transformers didn't exist (until the movie adaptation introduced Arcee in the US).
In the Marvel UK comics, Arcee was created as a female Autobot to appease angry feminists on Earth who thought the TFs were sexist for not having any females in their group. (A silly plot point, but considering some of the things certain feminists do/say, it's not quite as silly as real life can be at times.) This is often pointed to as evidence that the Transformers do not have gender and is correct -- if you're writing strictly comics-based fics.
In post-G1 continuities, female characters appeared with greater regularity and as regular series characters (the original female Autobots (Elita-1, Moonracer, Chromia and Firestar) appeared for one episode. Arcee was a regular character in G1 Season Three. Beast Wars had Black Arachnia and Airazor. Beast Machines had Black Arachnia, Botanica and Strika, who was a drastic design change from the pastel, roboticized human women of G1 as well as Black Arachnia's slinky sexiness. Strika was a big ass tank and larger than her male consort, Obsidian.
Further gender confusion comes from the fact that in at least two cases, characters who were voiced by female voice actors in the US were voiced by male voice actors in Japan: Airazor from Beast Wars (which made the character's romantic relationship with Tigatron a bit problematic) and Override from Cybertron/Galaxy Force (where the character is known as Nitro Convoy)
III. Romance:
The second thing to be aware of is that canonically, Transformers have also been shown in romantic relationships in just about every version of canon there is.
In the G1 cartoon, Optimus Prime and Elita-1 were dating back when they were still Orion Pax and Ariel; in the episode "War Dawn" Optimus (then still Orion Pax) specifically refers to Ariel (the future Elita-1) as his girlfriend.
Not a Romantic Relationship, but Octane was shown looking at an image of a female robots in "Starscream's Ghost."
In the manga version of one of the Japanese series, Transformers: Victory, Deathsaurus has a wife, Esmeryl and children (more on that further down).
In Beast Wars, Silverbolt and Black Arachnia had a thing for each other; as did Airazor and Tigatron. Beast Machines continued the Silverbolt/BA romance.
In Beast Machines, Obsidian and Strika refer to each other as 'consorts'; Rattrap and Botanica develop one of those stereotypical 'antagonistic but it turns into love' relationships (which, technically might also count as interspecies since he turns into a rat and she turns into a plant)
In Robots In Disguise (RiD), Side Burn has a thing for red sports cars -- as in, he likes to chase after them loudly proclaiming his love of them. This would probably work better if the sports cars he chased were sentient, as all he succeeds in doing is freaking out their human drivers.
In the Transformers Energon toyline, the character Downshift has an unnamed female 'life-partner.'
In Cybertron, Thunderblast made goo-goo eyes at whoever was currently leader of the Decepticons, though that seems to be driven more by her mercenary desires than by any sense of romantic interest.
A. Interspecies Romances are also fairly common in various canons. Most are male Transformers (generally Autobots)/human(oid) female, particularly in the G1 cartoon and comic series and in some cases the romance seems to be more a case of characters being fond of each other rather than hot, sweaty robo/human sexx0rs.
Examples: -- not a complete list. See the TF Wiki article above for a more complete list.
Seaspray/Alana -- "Sea Change" (G1 US Cartoon)
Powerglide/Astoria Carlton-Ritz -- "Girl Who Loved Powerglide", (US G1 Cartoon)
Skids/Charlene -- Transformers #20: "Showdown" (US G1 Marvel Comic)
Jazz/Marissa Fairborne -- US Dreamwave Comic
Ultra Magnus/Cindy Newell -- UK G1 Comics
Starscream/Alexis -- Armada/Energon/Cybertron continuity.
The Japanese toyline/radio drama, Transformers: KISS Play is based entirely around the idea of human girls kissing Autobots to give them special powers. Proving once again that Transformers canon can trump damn near anything the fans come up with for weirdness
B. Slash is technically canonical -- Mirage's infatuation withMegatron Galvatron in Energon is one example, but hardly the only one. A pair of Powermasters in one of the Japanese series also allegedly appeared to have a homosexual relationship but a) I can't find a reference to back this up and b) the Powermasters in question were technically humans in armor, rather than their robotic partners.
Considering Transformers is first and foremost a children's toyline, the chances of ever seeing openly gay characters is slim, but subtext exists like whoa. Like other fandoms, fanfic pairings tend to center around 'pretty' characters -- usually jets and sports cars -- and follow many of the usual tropes of slash fanfic, only with giant robots (or sometimes, giant robots and humans in the case of fanfic based in the live action movie 'verse).
C. Asexuality It's also worth noting that Cloudburst, an Autobot Pretender, did lay the groundwork for future internet arguments that Transformers cannot and do not have genders or romantic entanglements of any sort back in issue #53 of the Marvel G1 comics when he said "You don't understand! Where I come from, there are no men, no women...no mates!" to the First One of Femax -- who was attempting to make him her mate at the time.

'Course, while Cloudburst was nearly getting his head lopped off for refusing the First One, his buddy Landmine was busy flirting with some of the First One's bodyguards. So somebody apparently didn't get the memo -- or Cloudburst just doesn't like bald chicks. Or Transformers, as a species, are just as complicated and screwed up about sex as humans are. The US/UK comics continuity is unique in that it is the only example of a Transformers continuity that makes a definitive stand about whether gender exists.

Landmine is that faint blue blob between the other faint blue blobs. He's bragging to the ladies about his prowess as a warrior.
All of this talk about gender and romance really just lays the groundwork for the idea that romantic/sexual relationships between Transformers are possible, at least in certain canons. (Sorry Cloudburst, but you appear to be a lone voice in the darkness; your fellow Transformers seem to be hooking up like it's going to be banned tomorrow). It doesn't do anything to explain exactly how said robots are actually getting their grooves on. Or why it happens.
IV. Transformers and Reproduction:
Being robots, Transformers don't have to have sex to reproduce (though some ficwriters do have Transformers getting pregnant) so presumably they don't have genitalia like humans do (more on this in a minute). In fact, according to canon, Transformers can reproduce in several different ways. The Transformers Wiki has a TF Reproduction list but personally, I find the Playground Psychotic's list to be more inclusive and it also includes some fanfic methods. My personal take on these lists is that Transformers can use any method they choose to reproduce (since that's what happened in canon anyway).
Child Transformers: Canonically, there have been a few instances of Transformers having what appear to be children -- i.e. smaller robots who act in ways that are similar to human children. What's problematic is the fact that there are many, many examples of Transformers who are small but who are clearly meant to be adults -- i.e. the Minibots, the minicons, the Micromasters, the Decepticon and Autobot tapeformers, etc. As well as the fact that, in the G1 cartoon and comic at least, newly made Transformers such as the Stunticons and Aerialbots come online and are immediately able to take care of themselves, even to the point of engaging in battle within a few hours of being constructed.
But, as with anything else having to do with Transformers, you can find canonical references that are contradictory and can be used to justify damn near anything you might want to write:
The best example of a child-transformer is probably Wheelie from Transformers: The Movie (1986), who is described as a 'wild boy' and a youth -- but unlike human children who need to be taken care of, Wheelie managed to survive alone and without assistance on Quintessa for an unknown period of time. His function is Survivalist.
Also in the 1986 TF movie, we see several small robots running around on Lithone and laughing with what are clearly children's voices. But their planet got eaten by Unicron and their species wiped out so...Unicron's a baby-eating bastard.
As I mentioned above, in the Victory Manga, there are references to Decepticon children. In fact, the Autobots stop fighting the Decepticons when they learn that the Decepticons have been fighting to protect their wives and families (including children) back home.
So if they don't need to do it to reproduce, why would Transformers have sex? The honest answer is, because ficwriters want to write about them having sex. Really! It's the same reason why fanfic writers do anything, from Harry snogging Draco to the algebraic permutations of Gundam Wing slash.
'Course, on the other hand, Transformers are shown to canonically have romantic relationships and where there are romantic relationships, so it's entirely possible that there is some form of intimacy. Naturally, since Transformers was originally (and still is) a children's toyline/cartoon/comic series, we're probably never going to see anything more intimate than a kiss, but as with any other fandom, give fans an inch and we'll have people having sex on it.
Seriously though, my opinion is that any race that can feel hate can feel love and have a need for closeness and might feel the need to express that closeness in a physical nature. Or just want to get it on for the sheer hell of it.
V. The Nitty-Gritty:
The answer as to how Transformers have sex really boils down to this: it depends entirely on the preferences of the author in question but there are two main schools of thought on the subject: Stickyfic and Plug&Play (P&P):
A. Stickyfic is a broad term that covers any and all fics where Transformers have sex in ways that resemble human/organic sexual intercourse or, in some cases, that are essentially human sex with the terms changed to sound more robotic (interface instead of penis; recharge port instead of vagina/anus). It also includes fics where the Transformers themselves are humanoid robots under their external armor complete with humanoid genitalia (i.e. if Starscream takes off all his jet-bits, underneath he's a grey-skinned humanoid robot with hair and a true penis).
In stickyfics, the "how" is really pretty self-explanatory. An interface works just like a penis (up to and including an equivalent to semen which may or may not have reproductive properties) and a recharge port works just like a vagina (usually self-lubricating) or an anus (lubrication optional).
Stickyfics have a reputation for being not very well written. My personal opinion is that most of the bad examples of this method humanize the Transformers too much -- if I can take your fic and swap Optimus and Elita (or Optimus and Ironhide) for Bob and Janet (or Bob and Tom) and the story still makes sense, I'm not reading a Transformers fic. I'm reading about two humans cosplaying as Transformers. I do not want to read about humans cosplaying as Transformers. I am, personally, also of the opinion that it is possible to do a full-out stickyfic well but that doing so would require a lot of thought, effort and justification.
On the other hand, Transformers are often humanized in various ways in canon. Characters routinely do things that seem strange and out of character for a non-organic species. Some official artwork shows Transformers with teeth and tongues so quasi-organic genitalia might not be that much of a stretch, particularly when there are canonical references to strip clubs in Beast Wars. (Rattrap talks about going to a bar on Cybertron where "the serving 'bots are walking around minus their torso plates, you know what I mean?")
B. Plug&Play, on the other hand, is a broad term that covers any and all fics where Transformers have sex in ways that either do not resemble human/organic sexual intercourse at all or that only vaguely resemble human/organic intercourse. Plug&Play itself can be divided into various preferred methods, including but not limited to: spark bonding (i.e. merging their 'souls' for a brief time), field interfacing (i.e. mingling energy fields), physically interfacing by the use of jacks and patch cords (much in the way you'd connect a VCR to your TV, only they're getting off on it), tactile stimulation (i.e. a glorified term for snuggling), direct manipulation of circuitry, etc. The possibilities are limited only by the imagination of the writer. (see below for a more detailed listing of P&P methods)
My personal take is that any or all of these methods can be used. It's all just a matter of preference, much like humans have preferences for different positions/body types/etc, but mileages may vary.
In these cases, the nitty-gritty of the 'how' depends on the method chosen. My personal preference is for tactile stimulation -- TFs get off by having various sensitive areas of their bodies stroked or touched. The sensitive areas in question can vary from Transformer to Transformer since body types vary so widely. On a car or truck Transformer, the wheel wells or interior of their cab might be a sensitive area. On a plane or helicopter, the wings or rotors are usually earmarked as being particularly sensitive. In a sense, erogenous zones for Transformers can vary from design to design rather than being standard like they are for humans.
Plug&Play fics, in my personal opinion, tend to be better written. I'm no doubt biased by the fact that I write P&P stories myself, but generally it seems to me that the people writing P&P stories put more effort into balancing the Transformers' similarities to humans and their differences and thus, their stories feel more like they've grown naturally from the canon rather than being shoehorned into it. But, as with any writing, mileages vary and being P&P doesn't automatically make a fic good anymore than stickyfic means automatically bad.
Special Mentions: These are methods that skirt the edges of Stickyfic and Plug&Play, making categorizing them difficult/problematic:
Holo!Sex: The idea of Transformers using holograms in general and specifically using holographic images to help disguise the fact that they are vehicles running around with no drivers is not new. The use of holographic drivers/pilots in the 2007 live-action Transformers movie has inspired a technique where in Transformers use holographic creations to have sex with each other (kind of like 3-D cybersexual role-playing) or with human partners.
"Humanizing" can take on a literal meaning, if the fic centers around the idea of a Transformer getting a human body. Canonically, this idea first appeared in the G1 cartoon episode "Only Human" where a terrorist uses 'synthoid technology' to create artificial human bodies for Arcee, Ultra Magnus, Springer and Rodimus Prime. (The opposite, a human getting the body of a Transformer, also showed up in G1 US cartoon canon in the episode "Autobot Spike.")
-------------
Addendums:
Addendum I: Plug and Play Methods: This is a by no means comprehensive list; most of these categories were taken from a discussion over at the Slash Haven at the Padded Cell from back in 2005.
Spark-Bonding Essentially the idea that Transformers can bond their sparks -- which seem to function a lot like a human's soul. This bond can, in some fics, mean that the TFs in question are bonded for life (a potentially scary prospect considering Transformers can live for millions and millions of years). Spark-bonding, in a non-sexualized context, has been shown in Transformers canon -- Airazor and Tigatron were merged into an entirely new being in Beast Wars in this fashion. Also, Ratchet and Megatron were kinda/sorta bonded with each other after they ended up smooshed together in the Marvel comics and shared a psychic (sorta) link with each other later on after they got their own bodies back (though, it should be noted that 'spark' is a Beast Wars/Beast Machines idea that has been retconned back into G1 canon).
Field Interface: The idea that Transformers have an 'aura' or 'energy field' that extends a short distance out from their bodies and that this field can be manipulated for their own pleasure.
Literal Plug and Play: Transformers have jacks and patch cords and can link to one another for pleasurable purposes.
Circuit board/Circuit-Style: Involves removing part of a TF's outer casing in order to directly manipulate the circuitry underneath.
Tactile Stimulation: Just like it sounds -- touching/snuggling/cuddling either in robot mode or alt mode. No special organs or fields required, just plain and simple human physical contact. Best exhibited in the issue of the Marvel G1 comic where Charlene is washing a very happy Skids (though, to be fair, probably a non-sexual context within canon).
Virtual Reality: Transformers are a highly computerized/mechanical species, so chances are they've developed some sort of virtual reality simulation that can be used for various educational, entertainment and pervy purposes (y'know, just like the Internet is).
Radio Games: Essentially, phone sex -- except with the possibility that the signals being sent back and forth might be being sent directly to the TF's pleasure center. Or the two TFs in question could be getting off on talking dirty to each other.
Vibrational Stimulus: Vibrations feel good. This can be a variation on tactile stimulation if the vibrations are being caused simply by one TF revving his/her engines against another TF. Or it can be a more complex method, depending on authorial preference.
Note: This has become something of a work in progress. Links and additional information are still being added as I think of them or as things are suggested to me in the comments. Any new additions after this will be tacked on as appendixes to the end of this post. (Actually, not entirely true, in most cases the references have been worked into the original text of the article.)
Updated 09/18/07
Note to Non-Transformers Fans: "Transformers" is a catch-all term for characters and media produced by Hasbro. The first Transformers toys came out in 1984 and over the last 23 years, there have been numerous and often conflicting variations of Transformers from the old-school, Generation 1 characters most commonly associated with the name to Beast Wars/Beast Machines, Robots in Disguise, Armada/Energon/Cybertron and the current live-action Transformers movie. There are versions of Transformers -- toys, comics and cartoons -- that were released only in Japan and comic versions that are exclusive to the UK. Essentially, Transformers is a buffet and fans commonly pick and choose only those aspects that appeal to them. You Harry Potter fans think you have it rough with movie vs. book? You ain't seen nothing yet.
The point of this essay isn’t to convince people that Transformers must have sex or even that Transformer sex is officially canonical; the point of this essay is to show that those fic writers who do write Transformers in adult situations aren’t pulling their ideas out of nowhere. Transformers having romantic relationships and/or sex is a fairly reasonable extrapolation from canon for several series.
Table of Contents:
-----------
I. Introduction:
Ok, I've written about this before but I can't find the thread it was in, so here we go again (note: this is very long winded because I'm trying to get this all out, save it and just post it as needed the next time someone asks; apologies for the Teal Deer):
The first thing to keep in mind is that the Transformers, canonically, are sentient robots. Unlike many other fictional robots, they are capable of thinking independently and experience an emotional range very similar to humans -- anger, hate, love, jealousy, desire, etc.
Non-fans/new fans may want to take a look at the Transformers wiki's article on Transformers for more information about Transformers as a species.
II. Gender:
In addition to being human-like in terms of intelligence and emotional range, Transformers are also, in many cases gendered beings since there are both male and female Transformers depicted in several different continuities (though in many cases, females are rare for reasons that have never been explored in canon, but which may have something to do with Hasbro's belief that female characters don't sell as well to the Transformers' target audience, little boys).
In the G1 comics, female Transformers didn't exist (until the movie adaptation introduced Arcee in the US).
In the Marvel UK comics, Arcee was created as a female Autobot to appease angry feminists on Earth who thought the TFs were sexist for not having any females in their group. (A silly plot point, but considering some of the things certain feminists do/say, it's not quite as silly as real life can be at times.) This is often pointed to as evidence that the Transformers do not have gender and is correct -- if you're writing strictly comics-based fics.
In post-G1 continuities, female characters appeared with greater regularity and as regular series characters (the original female Autobots (Elita-1, Moonracer, Chromia and Firestar) appeared for one episode. Arcee was a regular character in G1 Season Three. Beast Wars had Black Arachnia and Airazor. Beast Machines had Black Arachnia, Botanica and Strika, who was a drastic design change from the pastel, roboticized human women of G1 as well as Black Arachnia's slinky sexiness. Strika was a big ass tank and larger than her male consort, Obsidian.
Further gender confusion comes from the fact that in at least two cases, characters who were voiced by female voice actors in the US were voiced by male voice actors in Japan: Airazor from Beast Wars (which made the character's romantic relationship with Tigatron a bit problematic) and Override from Cybertron/Galaxy Force (where the character is known as Nitro Convoy)
III. Romance:
The second thing to be aware of is that canonically, Transformers have also been shown in romantic relationships in just about every version of canon there is.
A. Interspecies Romances are also fairly common in various canons. Most are male Transformers (generally Autobots)/human(oid) female, particularly in the G1 cartoon and comic series and in some cases the romance seems to be more a case of characters being fond of each other rather than hot, sweaty robo/human sexx0rs.
Examples: -- not a complete list. See the TF Wiki article above for a more complete list.
B. Slash is technically canonical -- Mirage's infatuation with
Considering Transformers is first and foremost a children's toyline, the chances of ever seeing openly gay characters is slim, but subtext exists like whoa. Like other fandoms, fanfic pairings tend to center around 'pretty' characters -- usually jets and sports cars -- and follow many of the usual tropes of slash fanfic, only with giant robots (or sometimes, giant robots and humans in the case of fanfic based in the live action movie 'verse).
C. Asexuality It's also worth noting that Cloudburst, an Autobot Pretender, did lay the groundwork for future internet arguments that Transformers cannot and do not have genders or romantic entanglements of any sort back in issue #53 of the Marvel G1 comics when he said "You don't understand! Where I come from, there are no men, no women...no mates!" to the First One of Femax -- who was attempting to make him her mate at the time.
'Course, while Cloudburst was nearly getting his head lopped off for refusing the First One, his buddy Landmine was busy flirting with some of the First One's bodyguards. So somebody apparently didn't get the memo -- or Cloudburst just doesn't like bald chicks. Or Transformers, as a species, are just as complicated and screwed up about sex as humans are. The US/UK comics continuity is unique in that it is the only example of a Transformers continuity that makes a definitive stand about whether gender exists.
Landmine is that faint blue blob between the other faint blue blobs. He's bragging to the ladies about his prowess as a warrior.
All of this talk about gender and romance really just lays the groundwork for the idea that romantic/sexual relationships between Transformers are possible, at least in certain canons. (Sorry Cloudburst, but you appear to be a lone voice in the darkness; your fellow Transformers seem to be hooking up like it's going to be banned tomorrow). It doesn't do anything to explain exactly how said robots are actually getting their grooves on. Or why it happens.
IV. Transformers and Reproduction:
Being robots, Transformers don't have to have sex to reproduce (though some ficwriters do have Transformers getting pregnant) so presumably they don't have genitalia like humans do (more on this in a minute). In fact, according to canon, Transformers can reproduce in several different ways. The Transformers Wiki has a TF Reproduction list but personally, I find the Playground Psychotic's list to be more inclusive and it also includes some fanfic methods. My personal take on these lists is that Transformers can use any method they choose to reproduce (since that's what happened in canon anyway).
Child Transformers: Canonically, there have been a few instances of Transformers having what appear to be children -- i.e. smaller robots who act in ways that are similar to human children. What's problematic is the fact that there are many, many examples of Transformers who are small but who are clearly meant to be adults -- i.e. the Minibots, the minicons, the Micromasters, the Decepticon and Autobot tapeformers, etc. As well as the fact that, in the G1 cartoon and comic at least, newly made Transformers such as the Stunticons and Aerialbots come online and are immediately able to take care of themselves, even to the point of engaging in battle within a few hours of being constructed.
But, as with anything else having to do with Transformers, you can find canonical references that are contradictory and can be used to justify damn near anything you might want to write:
So if they don't need to do it to reproduce, why would Transformers have sex? The honest answer is, because ficwriters want to write about them having sex. Really! It's the same reason why fanfic writers do anything, from Harry snogging Draco to the algebraic permutations of Gundam Wing slash.
'Course, on the other hand, Transformers are shown to canonically have romantic relationships and where there are romantic relationships, so it's entirely possible that there is some form of intimacy. Naturally, since Transformers was originally (and still is) a children's toyline/cartoon/comic series, we're probably never going to see anything more intimate than a kiss, but as with any other fandom, give fans an inch and we'll have people having sex on it.
Seriously though, my opinion is that any race that can feel hate can feel love and have a need for closeness and might feel the need to express that closeness in a physical nature. Or just want to get it on for the sheer hell of it.
V. The Nitty-Gritty:
The answer as to how Transformers have sex really boils down to this: it depends entirely on the preferences of the author in question but there are two main schools of thought on the subject: Stickyfic and Plug&Play (P&P):
A. Stickyfic is a broad term that covers any and all fics where Transformers have sex in ways that resemble human/organic sexual intercourse or, in some cases, that are essentially human sex with the terms changed to sound more robotic (interface instead of penis; recharge port instead of vagina/anus). It also includes fics where the Transformers themselves are humanoid robots under their external armor complete with humanoid genitalia (i.e. if Starscream takes off all his jet-bits, underneath he's a grey-skinned humanoid robot with hair and a true penis).
In stickyfics, the "how" is really pretty self-explanatory. An interface works just like a penis (up to and including an equivalent to semen which may or may not have reproductive properties) and a recharge port works just like a vagina (usually self-lubricating) or an anus (lubrication optional).
Stickyfics have a reputation for being not very well written. My personal opinion is that most of the bad examples of this method humanize the Transformers too much -- if I can take your fic and swap Optimus and Elita (or Optimus and Ironhide) for Bob and Janet (or Bob and Tom) and the story still makes sense, I'm not reading a Transformers fic. I'm reading about two humans cosplaying as Transformers. I do not want to read about humans cosplaying as Transformers. I am, personally, also of the opinion that it is possible to do a full-out stickyfic well but that doing so would require a lot of thought, effort and justification.
On the other hand, Transformers are often humanized in various ways in canon. Characters routinely do things that seem strange and out of character for a non-organic species. Some official artwork shows Transformers with teeth and tongues so quasi-organic genitalia might not be that much of a stretch, particularly when there are canonical references to strip clubs in Beast Wars. (Rattrap talks about going to a bar on Cybertron where "the serving 'bots are walking around minus their torso plates, you know what I mean?")
B. Plug&Play, on the other hand, is a broad term that covers any and all fics where Transformers have sex in ways that either do not resemble human/organic sexual intercourse at all or that only vaguely resemble human/organic intercourse. Plug&Play itself can be divided into various preferred methods, including but not limited to: spark bonding (i.e. merging their 'souls' for a brief time), field interfacing (i.e. mingling energy fields), physically interfacing by the use of jacks and patch cords (much in the way you'd connect a VCR to your TV, only they're getting off on it), tactile stimulation (i.e. a glorified term for snuggling), direct manipulation of circuitry, etc. The possibilities are limited only by the imagination of the writer. (see below for a more detailed listing of P&P methods)
My personal take is that any or all of these methods can be used. It's all just a matter of preference, much like humans have preferences for different positions/body types/etc, but mileages may vary.
In these cases, the nitty-gritty of the 'how' depends on the method chosen. My personal preference is for tactile stimulation -- TFs get off by having various sensitive areas of their bodies stroked or touched. The sensitive areas in question can vary from Transformer to Transformer since body types vary so widely. On a car or truck Transformer, the wheel wells or interior of their cab might be a sensitive area. On a plane or helicopter, the wings or rotors are usually earmarked as being particularly sensitive. In a sense, erogenous zones for Transformers can vary from design to design rather than being standard like they are for humans.
Plug&Play fics, in my personal opinion, tend to be better written. I'm no doubt biased by the fact that I write P&P stories myself, but generally it seems to me that the people writing P&P stories put more effort into balancing the Transformers' similarities to humans and their differences and thus, their stories feel more like they've grown naturally from the canon rather than being shoehorned into it. But, as with any writing, mileages vary and being P&P doesn't automatically make a fic good anymore than stickyfic means automatically bad.
Special Mentions: These are methods that skirt the edges of Stickyfic and Plug&Play, making categorizing them difficult/problematic:
Holo!Sex: The idea of Transformers using holograms in general and specifically using holographic images to help disguise the fact that they are vehicles running around with no drivers is not new. The use of holographic drivers/pilots in the 2007 live-action Transformers movie has inspired a technique where in Transformers use holographic creations to have sex with each other (kind of like 3-D cybersexual role-playing) or with human partners.
"Humanizing" can take on a literal meaning, if the fic centers around the idea of a Transformer getting a human body. Canonically, this idea first appeared in the G1 cartoon episode "Only Human" where a terrorist uses 'synthoid technology' to create artificial human bodies for Arcee, Ultra Magnus, Springer and Rodimus Prime. (The opposite, a human getting the body of a Transformer, also showed up in G1 US cartoon canon in the episode "Autobot Spike.")
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Addendums:
Addendum I: Plug and Play Methods: This is a by no means comprehensive list; most of these categories were taken from a discussion over at the Slash Haven at the Padded Cell from back in 2005.