It very much depends on the group, and the player.
IMXP, it's the challenge of the "role-playing" part of it. If you're a skinny white guy who goes to audition for the role of "obese oriental woman", chances are people will raise eyebrows and you won't be able to do it well...you have no experience with something like that, and can only really use stereotypes.
Of course, in an RPG, stereotypes are really all we play as....dwarves are drunk and militaristic, elves are l33t ninj4, hobbits are homebodies, paladins are crusaders, druids love nature, blah blah blah, the game is *based* on stereotypes (or "archetypes" if you want to be PC about it).
And there are certain stereotypes for the game that subsume gender...I'd trust nearly any woman to play a raging orcish barbarian man, for instance, and I'd trust nearly any guy to play, I dunno, the aloof, unavailable elf princess role. Of course, if the woman insists on playing the orc as a leather-daddy who talks dirty to his enemies, or the man insists on playing the princess as a sex-crazed lesbian, those are things that break the mood too much for my tastes, and, frankly, reek of being tasteless. I don't care how 'in character' the story of how your hooker-with-a-heart-of-gold-sorceress is...there's just certain things that aren't apt for any game I'm willing to run.
That's not to say I'm not flexible....I've had characters who pledge themselves to fertility goddesses, or psions who really play up their Charisma. You make a character based on an Artemis figure of a forested virgin, that's cool, and you're welcome to make her gender and romantic affairs a big issue (and if it is a big issue for your character, you should expect it to become a big issue for the story, too). But crossing gender lines stretches the shared fantasy of a D&D world pretty thin, often to the point of breaking if not done VERY carefully. You've got to keep in mind other players, how well they'll buy that you are in the character of this woman. Generally, that's easier online than it would be in person.
But really, like a friend of mine said, we play this game to be someone different than ourselves. The standing rule probably isn't a good idea, even if the nervousness is understandable.