I have no problem with people playing opposite genders as long as they're serious about it, and roleplay the character in a realistic and believable fashion. My group is quite large, over thirty of us all together (not all of us play together at the same times, most of the time, though it has happened a few times, of course), and they're all exceedingly talented roleplayers, and take it quite seriously. Our group is roughly 50/50 on gender, not exactly, but almost half of them are women. I've also found and known many female gamers in my time. You'll find an enormous number of them online in online chat rooms that cater to roleplaying, for example, as well.
I have played with literally hundreds of different people over the thirty five years I've been roleplaying (you guessed it, I stared with the Basic D&D boxed set in April of 1974), and those players have run the gamut from extremely casual to extremely talented and into it, and everywhere in between.
Most of those I've played with that have played opposite genders have done so with varying degrees of success. Some have been eeriely convincing, and some have been absolutely disasterous. That being said, I guess it depends on the individual player and circumstances whether or not it bothers me.
It doesn't hurt having my kid sister, the shrink, playing to get all sorts of psychological insight into such matters in the group, either. *chuckles*