I definitely agree that cheating doesn't always result in a bad situation, and yes, people are human, they make mistakes. That said, the act of cheating which is in essence dishonesty, I can't say is right. I think that if someone does slip up (and I know I have in a past relationship), you should deal with it as constructively as possible and hope something good can come of it. A good result doesn't mean lying to your partner was okay, but it does show that you can handle things as a mature responsible adult and resolve things.
Personally though, I wouldn't want to be a part of that, even if it does result in a happy ending. I'm sure for everyone who helped somebody cheat and it ended well, there are a whole lot more who played a part in ruining a relationship. Plus, if I'm going to care for someone and get involved with them in any way (sexually, romantically, etc), I want to know they are an honest person. If I got close to them by them cheating on another partner, I don't think I would be able to trust that they would not do the same to me, and thats a risk to my emotional health and sexual safety that I just don't want to take.