Well, heres the thing about that, I could complain that something is too straight and stuff. There is a reason for such existing, because gay people exist just like straight people do. There is gay stuff because people want to be represented, and some are tired of the larger amount of heterosexual type games. Gay, bisexual, trans, all that representation is because those people exist. And like straight people, some want to be able to feel represented in something like a show or game. Not sure what I am saying makes much sense, but basically, it's because representation matters.
Edit: Basically, those themes exist as for representation, although there's quite a lot of gay games, I suggest comparing the number of straight games there are compared to gay, bi or trans, or LGBTQ themed ones entirely.