It's chemical/hormonal. Whether it's genetic or not is a matter of debate. Personally, I think it's more likely deveopmental in the womb. Regardless, none of it's cut & dry or definable. It's a matter of degrees across the board, with all kinds of social interference that changes the dynamics with each individual. If you put a dozen transexuals in a room, no 2 of them will agree on exactly what they're going through. Now multiply that by a couple billion people of every conceivable bent...
Butt what the hell. Carry on. Maybe we'll all fully understand everybody else & be singing Kum-bi,y'all by page 27.