On the LGBTQ/BL side of things, one I'd say is very over used is the gay guy hopelessly in love with his straight friend, and after a bunch of angst and awkward moments or maybe a night of drinking they finally "do it". Invariably the straight friend suddenly realizes that, hey, he likes it, and so now they are a couple but they take it reaaaaally slow because the previously straight friend isn't quite sure exactly how far he wants to go. Naturally, the relationship must be kept a secret from all the other friends, who know anyway.
I mean, yes, I can see the appeal of this kind of story. It gives hope and sunshine to the millions of gay people who have had crushes on their straight friends. But it also perpetuates two dangerous myths: First, that gay people are trying to "turn" straight people, and second, that those straight people can be "turned" (IE it makes it seem like being gay is a choice and you can turn it on or off).
I should point out that I do NOT mean stories in which a character confesses to another and discovers that the character is also secretly gay himself. That is something that actually happens. What I'm talking about is when a gay character kisses a straight character and suddenly that straight character isn't so straight anymore.