On crossplaying, I tend to ask people to play their gender IRL. I do the same thing, simply because when you hear a deep, bassy man voice playing a gemale character or the opposite, it pulls me out a little.

I certainly wouldn't outright forbid it, but it'd be a "If possible, you're a dude, please play a dude or get real good at female voices." (Note: I've never gamed IRL with someone who was assigned a gender different to their actual one, so that's never been an issue for me. Were it to come up, I'd stick to the guideline-if you're a lady, play a lady; if you're a dude, play a dude; matching their ACTUAL gender, not assigned.)

In PbP games or text-based games or in any other type of game where I can't hear voices, go nuts. It doesn't affect my suspension of disbelief in the slightest if there's a woman behind the keyboard typing out a man's words, or vice versa.