Optional ♂ or ♀

[Mirage]

Yes, this seems entirely realistic. It probably won't cost more than a few cents per user!

On a serious note:
No, OP, this idea is ridiculous. Either assume all players to be male until proven female, or treat people as their in-game gender. It's your choice.

Personally, I treat female characters as females unless they ask me not to. That doesn't mean I give them free gear, however.