I think it is really difficult on a game like that. Like…I glanced at it because I like sex as much as the next person, but (happily!) a lot of the canonically LGBT characters I might have wanted to play were already taken.
Under their rules, I think I could take a canonically straight character and gay them up, but that feels weird - especially on a sex-focused game.
Like, if I played the gay Scott Summers and Jean Grey showed up wanting snoo-snoo, I think I would kind of feel duty-bound, especially since they are such an iconic pairing.
Obviously, I would not be, and their rules make that clear, but I think the weight of social expectations would certainly be there, and I would feel like I was taking something away from that other player. Maybe that is a weird hangup only I have.
I am sure it is worse for anyone who would want to play a trans character because I can’t even think of one (granted, I am not a huge comics person).
I do not think it is sufficient to say, “You could play a shapeshifter character, who can be any gender,” because unless the character is also trans, it is not equivalent.
It seems like a really difficult knot that could be easily untangled by allowing OCs.