I don't really understand it either, but I think I may know part of the reason we keep asking this question: When we create characters, there are some of us (myself included), that project ourselves on our character -we see the character as ourselves. I think there is a minority (just seems there are fewer to me, but still a significant number) of players who are detached from their character, and see it as a individual outside themselves. Those are your male/female crossovers. Those players like to build characters totally inverse of who they are. Then there's probably trans-genders here and there that identify with the opposite sex. But much like real life, they are truly low-frequency.