Dude, Girls are overrated. *activates flame-retardation skin*
If she's going out with this one guy and kissing him and everything, and she comes back to you and says that she didnt mean for it to happen, and she goes out with him AGAIN, well doesn't that tell you something about her?
Seriously, one of two things needs to happen right here and now. Either Step up to the guy and "inform" him of the situation, however you see fit, wether you talk to him, or knock him out over it. I honestly have seen this happen way too many times, and all I can say is that girls like these are more trouble than they're worth. The other thing is to get rid of her - save the stress.
Ask yourself, if she says she wants to be with you yet keeps going out with this guy who kisses her and such, ask yourself if you are honestly willing to put up with this kind of wishy-washy behavior. Is it honestly worth her goofing around with other people, when you're honestly working hard to be faithful to her? If she's not going to be faithful, and go around with other guys, and NOT with YOU, where she should be in the first place, why should you put forth the energy and stress to save something that she ovbiously doesnt value.
BTW, You're in Utah? So am I. Trust me when I say that I've seen girls act like this around here. Wether you're from the north or the south, they're typically the same. Ahh.. if I were to move back to Nevada..... mm...