cheating is a cry for help or understanding. I think a one off incident should be forgiven, talked about and truths be told. There's no cheating when there is a solid relationship. It's a symptom of unhappiness or a momentary lapse. People are weak, make mistakes, get caught up in a moment or are duped.
Once is forgivable, but then the real work of looking at the root cause has to begin. It's hard, and painful but ultimately repairable, if there is real love.
A second time, is the sign to call it quits.
just my opinion... :-)