Breja: Maybe I just had bad luck then. I don't mean to hate on female players and it's not meant as some "women stay away from RPGs" manifesto. I'm just speaking from my experience (such as it is, not even close to yours).
I didn't take it this way ;-)
I guess it's just that - bad luck. Either the female players just weren't very good or your male players were of the "Gentlemen's club" kind - not being able to include a woman in the play very well. From my experience all-male groups tend to play more violent and raunchy which might turn off female players. Although the raunchiest party I ever DM'd actually had two women (and 2-3 guys)...
Breja: I always find that awkward, though usually it amount to nothing really. The last time I had a player do that (and he's a really good player overall, one of the best I know), the end result was that him playing a female character barely ever came up, he didn't really role play it in any way whatsoever, and we all kept forgetting his character even is a woman. I think he made the decision to play a woman on a whim, but didn't really see much point in it later.
I've seen good examples of male players playing female characters, although it's really rare. For good role-playing in this case it's necessary that the player has a very clear image of his character. Playing your own sex/gender you can have your character grow to you more... subconsciously. You play and add details along the way from your guts. Playing "cross" means you have to more actively think about detailing the person you're playing.
As DM I of course do gender crossing all the time, because I play all the NPCs. Some of them are really fleshed out and are recurring (sometimes over real years of playing...), some of them join the party for a quest (like they're my PC for a few evenings). I know that I do put a lot more effort into female characters beforehand and while playing to make them believable and "alive" - and according to my players I'm doing a pretty good job there. They still talk about some of my "creations" after years, like they had met them in the flesh.
On the other hand female players playing male characters are mostly really good... I don't know why that is - maybe we men really are more simple ;-)
What baffles me is that I have never ever met a female DM...
dtgreene: I can think of one very good reason to allow gender-crossing. A small (but nonzero) percentage of the people who play tabletop RPGs are transgender, or suspect they might be; playing a character opposite their apparent gender, for those players, can be a good way to explore their gender identity and to relieve dysphoria.
I know that some DMs do not "allow" gender-crossing - probably from bad experience - I don't know. In my case I really don't put limits on the players. I want good role-playing from my players and a badly played female character is not worse than a badly played elf (see "human with pointy ears"). If the character does not work I'll have a word with the player, and if nothing else helps the character will die in a graceful, heroic way and a new one is created.
That said, I've never actually had a transgender player. I've had gay, lesbian and bi-sexual though ;-)