(This is more of a problem with Yaoi than BL) Self-lubricating assholes. Every time I've seen it my mind goes to watery diarrhea, and I can't unsee it after that. Truly disgusting! I can only imagine the smell.
All the chicks are either antagonists or extremely disrespectful fujos. I feel particularly bad for the antagonist women because 90% of the comics they're in it turns out they used to be the love interest's partner, who gets cheated on. While they usually show the dude is miserable with her, or the story treats her like a throne, it's shitty that the guy just doesn't break it off with her until after she find out he's cheating.
Like, that's a whole ass fictional depiction of a human being treated like crap to make the main relationship look good. How garbage is that!
Other than these two I don't necessarily have a problem with the subgenre, my beef tends to be with how the creators treat others, which has gotten much better over the years. A lot less cis/het teenage girls sexualizing real gay men, or shipping real people like objects than when I was a teen.
Sure, some tropes can be gross, like the SA stuff. Although I don't want to judge the work since the person could be dealing with trauma via their stories. (Even the romanticized SA stuff can be therapeutic for some survivors since they're still taking control over the situation via the fiction.) Children and people incapable of differentiating fiction from reality definitely shouldn't consume that stuff, because they will probably take it to heart, but most adults who like weird/gross art/fiction don't tend advocate for it irl. As long as the creator isn't a rl rape/abuse/sexism/etc. apologist, or isn't a freaken monster themselves, I couldn't care less.
... Admittedly I'm probably a little bias since I am one of those creators making gross fiction. Lol
Bonus romance peeve that isn't just used in BL: Love triangles. Why can't the three of them just hook up and have a nice poly relationship? Especially when they all have good chemistry with one another.