So, I don't mind things like stalkers or master/slave or just generally messy toxic bad relationships in any kind of romance with any type of pairing, my issue is when the author doesn't seem to realise that's what they're doing. If you want to read a dark romance bordering on thriller about power imbalances, that's cool, sometimes I enjoy that too, but like I want to seek that out and read it. I don't want my wholesome relationship story to actually be a dark mess. And I think that's the big problem with a lot of romances, especially older BL but not exclusive to it. There's nothing wrong with the content in itself, more that you're selling it as a thing it's not and don't seem aware that it's not.
More generally, the love interest being a nothing character. We all know this is why BL ships happen in a lot of fandoms because the female love interest has no time on screen with the main character and no chemistry vs the rival/best friends who has seasons worth of content and chemistry on their relationship. I don't care if the relationship is gay, straight, open or polyamorous, I just want they to have chemistry and all the characters to be actual characters.
Any character who's not the love interest is automatically a threat. Especially common with women. If there's a woman who isn't the main girl/boy, she a bitch here to steal your man! Every much in line with the I'm not like other girls vibe lots of YA books had and still do have to an extent.
They got together in high school and they're together forever. Like, I get it, teenagers think like that, their romance is the greatest love story of all time and Romeo and Juliet is a love story and no one has ever loved like these two 14 year old morons who've not experienced life, but sure. And yeah, some high school couples do last. But when every couple who got together at like 18 or under is happily married and together like 10 or 15 years later? It just makes me roll my eyes.
But on the flip side, characters who randomly got together off screen during a time skip. Like, ok this is very specific, but if you two named characters who're part of the main cast and then pair them together off screen during a time skip for your epilogue it just looks to me like you didn't want to go through the effort of writing the relationship. At the very least, let them have some chemistry before the time skip. I'd rather you just tell me they have a girlfriend but I don't know her, then shove them together where I couldn't see it happen and expect me to buy it.