To be fair, I don't think it would mean you were gay or anything at all. It would only make you gay if it was, or you were hoping it was, a guy ramming his shaft up your arse. If it's a lady doing it, then you're still hetrosexual. The mere act of having something stuck up your arse isn't inherently gay, in fact I guess it's a pretty logical thing to do being that your g-spot is up there. Fucking nature.
Plus if a girls gonna do that it normally entails some kind of mega bonus for the guy in question. This all being said bum sexing is, from what I've gathered, an entirely messy affair, so most likely no.