thats crap - take advise from a woman, the only reason she told you about what happened is because she felt GUILTY and she had to fess up. any girl who REALLY cared about you would not do that to you.

you trusted her enough to go out will her friends and guys without you, and look how she repaid you. would you ever do that to her? its a complete lack of respect. ok everyone "makes mistakes" but a mistake would have been that she kissed him, once, then told you about it, not a full-on makeout session.

for all you really know she could have already slept with him, and you would never have known the difference because now she feels better that she "told you what happened"

i dont know all of the details of yalls relationship, but i know damn well that if my bf of 2 years ever pulled that, he would be gone in a heartbeat, and if i ever did that to him, i would fully expect him to dump me. there are some lines you just should not cross in any relationship, she doesnt really love you man, she is just used to having you around.