I think cheating is a simple definition. If you are in a relationship with someone, and explore a relationship with another on any level of intimacy (dating, kissing, sex) without your partner's knowledge or consent, it's cheating. However, the lines of whether an instance of infidelity was right or wrong, and who is to blame, are certainly up for debate and interpretation. I'm not going to begrudge a person who cheats on the path to leaving an abusive relationship. I do not, however, agree with a person who will take any opportunity they are away from their partner to sleep around just for the fun of it.
I don't believe everyone cheats, but I also believe there is a grey shade to everything. I met my husband while he was married. At the time, she was in a psychiatric hospital, and their marriage had been dead and abusive for years. So yes, he technically cheated. But, I can't say it was wrong. I don't in any way shape or form believe that cheating is part of his character, or that on a whim he is going to go bang a stewardess. I do however, believe that if we made it to a point in our marriage that I was forcing him to find love and companionship with someone else, then I'm the one that screwed up somewhere.
Hooray for ideas and discussion! Great question, CarrieAnn!