I've got no problem with boys wanting to be girls or vice versa. However, I would like to ask why? Is it a bit of role-playing or is it a strategy to soften the response of another player in the game interactions?
I'm not criticing, I'm only curious (about wanting to be female)!
Well, there are loads of reasons.
1. Usually female characters in games are focused on speed and technique, whereas male characters tend more towards power and defence. Less so in MMOs, there's usually no difference there.
2. In most games, your character is placed first and foremost in the middle of the screen. If you're going to spend most of your time staring at your character's backside, it may as well be a female backside.
3. MMOs have a large and varied audience, but at least part of this audience is horny adolescent boys. If they think you're female, they'll give you all kinds of items in exchange for continued conversation.
Of course, some see themselves as female in real life, or wish to be female in real life, but the vast majority have other motives.
Personally I'm not too fussed about whether I'm perceived as male or female, and given that I'm speed-oriented (and maybe the backside thing comes into play a little bit), then it really has to be female.