why the heck so many ppl here repeat over and over the thing about 'exclusivity'?? when you start to date a girl you're not signing a conctract.
It is common sense that if you're seriously dating someone and you care about her, you see each other almost everyday and so on, YOU DO NOT FUC-K OTHER PEOPLE, you dont need to have a conversation about it! you can have it, but after a few weeks, having sex and acting like a couple you do not , I repeat, you do not have sex with other people if you want this to be serious.
You do not need to sit and say: "hey, sign this ok? this way we agree that we're not gonna fu-ck other people ok? I'll bring my lawyer"
it seems they were dating three months when she decided to f-uck that other dude. I dont know how serious you were by then, but if you were, she's a damn sl-ut, and you should dump her. If, however, you were not that close and not seriously dating...well, in any case a woman that has sex with two men during the same period of time is a wh-ore in my opinion and not worth of a serious relationship.