People assume I'm male because I play a large-breasted catgirl and like to dress her in skimpy outfits. But I'm female.

I generally assume everyone is the same gender as their character until told otherwise. This is a role playing game, after all. If I'm talking about a character named Barbie Sparkles wearing a pink Spring Dress, I'm going to refer to that character as "her" or "she" because clearly the person controlling that character is comfortable role playing as a female character. If I later find out that the person behind Barbie Sparkles is a man, then I'll refer to him as male in chats or whatever.

Sometimes you can't discern a person's gender from their name. If the character is a Lalafell, depending on what they're wearing, it's also difficult to determine their gender, since both male and female Lalas are built the same. If they're in full plate mail or a robe with a cowl, it's pretty much impossible to tell. I generally default to "she" in these situations. Probalby because I'm female.