Boring protagonists without any personality whats-so-ever. They have no achievements, no morals, no humor and no intelligence at all. They simply exist for us to control like a puppet. I don't want a fully autonomous protagonist, but I do like them to have SOME personality.
Degrading a protagonist... Whats the point? Self-deprecation is fun in moderation, jokes can also be fun. But why do some stories try to do it at every convenience possible? It's always too much, makes some characters unlikable and generally only highlights the problems from the last paragraph.
Protagonists overreaction to nudity or beauty. The whole "I can't look away", peeking in showers, peeking everywhere, it's kinda ridiculous and creepy. I didn't even act like that when I was a 12 year old virgin, forget 18+. Even the 18 year old virgins I knew weren't this creepy... It's one thing to check someone out, its another to freak out because you see a hot girl.
Overly perverted protagonists. See paragraphs one and three.
Oversized tits, ass, dicks, etc and overly aggressive or bitchy characters, primarily of the mother and/or sister variety. Should be stated as well.
"Forcing Relationships" by using other characters or events which only exists for the sake of forcing a sex scene or forcing a relationship to the next level. These things never go well, and simply serve as a cheap and easy way to speed up the story. The relationships are generally progressing fine on their own, why force them? Hell, your the dev and the writer. You can increase the pace whenever you want, so why force it?
"Incest" that's barely incest. I don't mind an incest relationship that develops quickly, but whats the point of adding an incest character that is already in a relationship with the protagonist before the story even begins? Just make her a girlfriend or friend with specific kinks or quirks that advances the story in the same way. At least that way she's unique.
Lesbian kinks being treated as a universal kink. Not all dudes are into lesbians simply because they're a dude.
"Unavoidable" kinks which aren't actually avoidable. Many unavoidable tags are simply hidden from the player but still happen according to the story and characters. Not exactly avoidable.