My ex cheated on me...eventually I forgave her. But not till after a long bout with depression. It was over as soon as she told me about it.
At the time when you first find out there is a tidal surge of uncontrollable emotions. First thought is kill both people involved. I had to walk away before I did something I'd regret.
So while it is forgivable, eventually. It is still a damn good way to end a relationship! Obviously who ever is doing the cheating doesn't give a fuck about the other person anymore. And doesn't have the balls to say so.
If your in an open relationship and it's under controlled circumstances, then it's not cheating in the first place. But if it's suppose to be just the 2 of you, then it needs to stay just the 2 of you. If either step outside of that, it's over, end of story!