I don't think there's any pat answer. You really have to look at the circumstances, if it only happened once. If it's not the first time, you may want to look at the character of the person and question whether it's the right relationship for you. I guess I would seperate for a while, and reavaluate our relationship. If I thought he was sincere I would give him a second chance.
Time does heal everything.
Marietta Ga. If he has ever even thought of cheating,chances are he will cheat. No Way, Don't play his game, BE Strong and move on.