It is really beyond stupid for people to assume that male characters are always male and that female characters are always female.
For the reasons you stated are the obvious ones. People want to play a character they LIKE. And well most of the Alliance male races look pretty silly so... You know...
One of my friends, who is a girl, won't even play the female characters. She plays a male Orc Shaman actually... We didn't really play together because she can't stand the female Alliance characters
