Generally speaking I think the problem isn't a bad plot or a bad story line so much as it is bad writing. There are story lines that seem bad because no skilled author would bother with them. The essence of story telling is making it believable. The purpose of story telling is to be entertaining (and to some extent educational).
An impossible to believe story line will lead to an impossible to believe story. And even if it could be made believable a story with a rape and torture and Stockholm Syndrome theme isn't going to be entertaining to enough readers to make it marketable. When a loving couple invites a third into their bed because one of them doesn't think the other has been a big enough slut in the past that's not believable. When two guys are totally hot for each other and neither one of them has any idea how the other feels that's usually pretty hard to believe. When a cis-gendered gay male and a trans-gendered gay man hook up and there's no drama beyond normal relationship hurt feelings or misunderstanding I personally think that stretches credibility a bit too much.
On the flip side I can believe that a couple can meet and have really rough sex which is mutually enjoyable and a relationship can develop. I can believe that a college jock would fall for a geek, a male cheerleader, or his coach. I can believe that two hot firemen can secretly long for each other and only act on it when something changes to force the reevaluation. And I can believe that no matter how "good" or "classic" the story line is that there's someone out there who could write the story and not only not make it enjoyable, but manage to make it unbelievable too.
Having a bad story line is just another writing error. It's like all the psuedo-foodie cozy MM romance authors (talk about niche
) who write chefs or bakers into their stories and then do stupid crap like putting rosemary into pesto or confusing meringues and madelines. It's a result of not doing enough research to make it believable and not writing about things they actually know about.
Not writing the characters well enough to make them real and/or likeable can make a story line seem bad. The idea that two men stuck on an island together might have sex isn't hard to believe. And if the characters are written well you could even believe it could become more than just sex. But if the characters are written as totally straight then there are too many barriers to belief for an informed reader to accept the story.
There are authors that write total tops who flip for their total bottom lovers to prove how much they love them. That bugs me because you obviously have an author who's clueless and still managed to get a book published. In that situation the total bottom probably wouldn't be able to get an erection anyway. And total tops are a psychological phenomena so they're more worried about proving things to themselves rather than a lover. Bad writing strikes again.
What I'm trying to get at is that I don't think there necessarily are bad plots. People are incredibly diverse and extremely hard to predict when it comes to sex and relationships. If the author can make the characters in the story believable and interesting that goes a long way to making the story believable and interesting. If the characters are two-dimensional, consistently make bad choices, are racist, bigoted, or have any other traits you wouldn't find desirable you're less likely to like the character and by extension the story. Applying these same traits to the antagonist can, conversely, make them less likeable which does a better job of selling the story.
That said I totally agree with dante's point that the blurb at the front of the book is
BULLSHIT and probably nor enough to protect the publisher in the event of a serious legal challenge. A responsible publisher wouldn't put a recipe for C4 in a spy novel and shouldn't put a recipe for disaster in a romance novel.
Some of the sessions you read in MM fiction are only believable if you're totally ignorant of what the BDSM community offers. In my experience there's always discussion ahead of time. Sessions that just happen are as realistic as trauma that magically disappears when you get a kiss from your prince.
Now if you really meant
turn off I'm going to have to go with FF, MF and MFF romances with lots of heavy duty sex scenes......