hahaha, i doubt anyone down with that, but who knows,a lot of "free spirits" around.
look, you like her and you are into what she's doing at the moment, that's the vibe you gave off. so, breaking up would be a shame! isn't there anything you dreamt of doing with a hottie and never had the chance to? i think this might be it. ask her if she likes other girls (3some!!:)4some, 5some, 6some! oh the opportunities! see it as an opportunity)
i had a fb that i suspected in being obsessed with fingering a guy! lol, not kidding

never talked about it, she never offered it. when we were in missionary, from time to time i felt her hand creep closer and closer lol. when it started getting to a point i was not cool with, i did what women do - gently placed her hand elsewhere. since women are often less persistent than men, that game continued for about 3 or 4 times. then it stopped. that's it.
alternatively, if you really feel you want to talk about this. i would casually bring up, "what's your wildest sexual fantasy?" or something like that. sound like it's just for fun. try to get her to tell you about it, not "let me show you" lol

if it's way off tell her, "hey, i'm not down with that shit, girl...." joking voice. make it a light convo. it's not a big deal.
do you really think she wants to shit on you? lol.