Generaly, it's sort of an unwritten rule in my gaming circle that you play a character that's the same sex as you are. Occasionaly, we let people stray from that with a good concept, of if for some reason it's not reasonably feasable. I have no problem playing a female character, but I rarely do.
Eberron might confuse things a bit, given that Warforged and Changelings don't exactly have a set race (Warforged tend to be closer to one or the other, but changelings can be both and function in every way as both). I'm sure there are other nuter or ambiguous races out there too.