speaking as someone who has been cheated on, and who has cheated on, cheating isn't a problem in itself.
People do not just go out and cheat on their significant other just because an opportunity presents itself. Infidelity is a symptom of a larger problem within the relationship, whether it's like dllavaneras said, or if it's any number of problems.
When i found out my girlfriend cheated on me, yeah, i was pissed off. I told her that i needed some time away from her to go and think things over and clear my head, but when it came down to it, i asked her if she even still wanted to be with me, what she thought was wrong with our relationship, etc, and if she still wanted to be together that we would work at it. I had the benefit of experience on the other side of this conversation.
Anyway, we ended up staying together and trying to make things work for another two months before ending things mutually.
I think the real problem is when your significant other doesn't respect you enough to tell you that they've cheated on you. That's when I don't think that i could forgive or take back.