Reading through the comments here is very interesting. Some sound quite indignant at the very mention of infidelity. In France mistresses are almost the norm. It has been accepted as a part of life. Could it be our puritanical history that colours how we feel in this country?
What is more important in a relationship, the failure of one partner or the unwillingness of the other to forgive and move on?
And which is worse, the broken promise, the deceit, or the actual straying? My ex told me on our honeymoon that he'd been with an old girlfriend since we got engaged. I was very hurt. Should he have told me? I'm not sure why he did except that he then asked if I'd been unfaithful to him. I hadn't, and never was, though he never believed that. I should have known then what I was in for, but I was only 17 and quite naive.
Until quite recently people fondly imagined that it was ok for animals to give in to their urges, but certain birds mated for life, and that was held up as an example to us all. Now that it has been shown that even those who do mate for life aren't immune to nipping in and out of each others nests, we'll have to rethink on that one. 