I used to be pretty balls deep into TF fandom, can confirm it's 99% faggotry like this. Some personal anecdotes I've come across:
I remember in my halcyon innocent days I came across a fic where Optimus gets raped by Ravage (AKA a fucking robot cat, so it was bestiality). Also it started with a declaration that Primus and Unicorn (robot god and satan) would have a big ol orgy every few years. Shit was weird and on FF.net, though I don't even know if it's still up.
Putting the literal "trans" in transformers, there's a metric shitton of fics where even if the porn involves female characters they end up having dicks (or spikes, or cords, or whatever the fuck). Why even bother having a girl when you're just turning her into a guy, retards.
Also, incest. Lots and lots of incest. Especially between Starscream, Thundercracker and Skywarp who are said to be brothers in a lot of fanworks (just so people can wank over them having threesomes).
The bullshit around 'Seekers' (fan term for flying robots like Starscream and his ilk) is a whole other pot of toxic sludge; they're basically fucktoys or fetish victims in any porn story featuring them. They might be the only ones who can do Mpreg, they might lay eggs, they might go into heat or whatever gets these weirdos off.
Sorry I can't provide specific examples, I'd usually just skim the freakiest fics I'd come across and then try to purge them from memory. But AO3 makes finding the trash a lot easier thanks to excessive content tags.
Best bet is that they're inter...sex...? I don't know, the most Transformers porn shit I saw was some dude's really wholesome Airachnid stuff, so I have no idea.
I could use a palate cleanser after dredging up all those cursed memories- wholesome Airachnid you say? I always liked her (mostly cause other people fucking hated her guts and it was funny to see). Though the only wholesome thing I've seen was one fic the length of a novel that heavily featured her and Optimus I think (never read it cause, yknow, time investment).
All the other ones seemed to just give her dicks. Especially one where she raped Bumblebee and then he got sex therapy from Optimus' cock. That one I do remember very well and could find the link for very eaily... but I don't know if I want to
