In all video games where it's allowed, I play my RL gender (female), but most men I know play the opposite of their RL genders. In SL that's a little different, but if a gendered male wants to play an SL female or SL TS to explore things in their own lives they can't live out normally (for what ever reason), so be it. I know several trans gendered, and gender queer people in RL that this doesn't phase me.
If you think about it, in real life there are six "genders" total. Male, Female, Trans-male, Trans-female, hermaphrodite (both genders) and then the gender absent. In some religions they break it down into 8 (male living as female, not identified female, and female living as male, not identified male), those are usually shamans or holy people.
SL gives people the possibility of who they dream to be. Some of us are happy on all levels and reproduce ourselves in game, other's feel the need to tweak, for personal or curiosities sake.