For some reason everyone's down voting the most sensible thing.
Which would be to dump her. I mean sure you can confront her about it first, but all you're going to hear is "It meant nothing." Which sure is odd, because if it meant nothing it never would've happened. So then you'll press her for details and she'll admit she was attracted to him or that she's feeling less attracted to you or some other run of the mill bullcrap a girl will feed you, but the fact of the matter is she was getting hit on, possibly felt up, and didn't reject the dude outright despite having a boyfriend. If his attentions had been unwanted she would've told him she had a boyfriend, and if he'd been forcing his attentions on her, she would've sought you out so that you could protect her.
So yeah, go ahead, have the conversation proper, and then when you feel the piercing knife of betrayal in your back, dump the bitch.
Or you can save yourself the pain, call her out on it and dump her without forcing the conversation and walk off in vindication knowing you were right to leave her ass. I've done both of these, and I can say the second one feels a lot better, even if there is the small chance it did mean nothing. If there's one thing you shouldn't tolerate, it's unfaithfulness.

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07 Oct 2013 1:23PM