I find all the irritation about this topic incredibly amusing personally. I have never been a part of any other game where this was such an issue. My personal reasons for doing it in this game are simple: aesthetics and variety. I wanted to have another Warlock, but didn't really want to have a carbon copy of my first one, so I decided to make a female one to mix things up. I find it far easier to make a female character look good personally. My only guess as to why ppl have a problem with others who play as opposite genders is because it messes with their perception of who the person behind the avatar is. (This is only a guess, not what I believe to be truth).