I think people can change, but that is often a really hard pill to swallow.
For example, my fiance was in a relationship with a girl for four years. She cheated on him quite a few times and he always took her back. He finally had enough, so before he dumped her, he cheated on her with one of her enemies. I think he was justified in cheating on her but then again, I think once someone cheats on you, to hell with them. If they want to screw up something special for a piece of ass, it's not worth it. I would dump anyone who cheated on me, regardless of what we had. I do believe in the good of people, but like I said, unless I was faced with that situation, it would be a really hard pill to swallow and one helluva situation to deal with.