i'd say if you're willing to share "your girl" with someone else, she's not really "your girl"
but who am i to argue with obibun of "look idiots i know everything about women i hang out near them" fame
You seem to know what you're talking about here, but also seem like the type who would be jealous if your girlfriend was hanging out with her friends instead of you.
I don't see a problem if my girlfriend wants to get sexual with another chick. For me, that's hot as shit (assuming the other girl is attractive). I don't really consider it cheating either, and would not be jealous - and maybe that's just genetic seeing as there's no dick in the equation ergo no children etc. etc. etc.
As long as I'm involved somehow, anyway. If she's just sneaking off in the night to be with some girl, I'd probably be pretty hurt by it. If she says, 'I really want to go down on that hot girl over there if you want to fuck me while it happens', I'd be game, sure as hell.