Well I'm a male IRL, and most of the time I play male characters. More so in an MMO environment than a single player experience. The reason I USUALLY play males in MMO's is because of a couple factors:
1. The character is my avatar, a virtual representation of myself, so I pick a male. Helps if I can make myself look like a bad ass. lol.
2. Female characters are guaranteed to get hit on by every other male in the game (which is about 90% of the population). Can be extremely annoying. I feel sorry for the real women who play these games :P
However, lately I've been playing female characters in WoW. Why you ask? Well, for one thing, it's something different. And I just find the female models to be more aesthetically pleasing (i.e. stylish animations, etc.) I am NOT one of those guys who sits there and strips his Night Elf naked to dance in Ironforge lol. When I say "aesthetically pleasing" I mean just that; the animations, the art, etc. But there will ALWAYS be other people who judge and don't understand and blame you for being homosexual just because you played a female character in an MMO. To these people, I'd like to ask if they ever played Tomb Raider, or ANY game with a female lead role, and ask them if they think playing that is homosexual behavior. Pure nonsense and absolute rubbish. But then, I really don't care what the opinion is of someone who is that immature.
For some reason, in WoW I find it harder to play females than in Guild Wars. In Guild Wars all my characters are female. Maybe it's the fact that WoW has more of a sense of a "World". I don't know the reasoning. In Hellgate London, my characters are female. Just some examples.
I once was a member of a guild in WoW who had a male character who was actually played by a female. And I can understand why women do this. The flirting and one liners really get annoying when you're playing. I mean come on people. We are here to play a GAME, NOT to pick up a date. If you are using WoW to meet women, you have some serious issues that need dealt with.
Some people just won't ever get it, or understand. But I'm fine with that. It's like a lot of things in life. The important thing is that I play the way I WANT to have fun, and enjoy myself.