Re: the ban rule
A simple question: Why should DMs have all the fun?
After all, unless all of the NPCs in the campaign are the DM's sex, the DM is engaging in precisely the same form of roleplay- admittedly abbreviated by necessity- that the players are barred from trying.
While I don't hold with the "no cross-gender RPing" rule (I'd say I play about 50% male, 50% female characters), this logic is not entirely valid. DMs get to roleplay dragons and demon lords too. Getting to play stuff that the players can't is one of the perks of the job.
However, most of the reasons for not allowing dragon or demon lord PCs don't really apply to cross-gender PCs.