I think if I were married, I'd have a stronger notion of infidelity. Being faithful in a marriage just seems right, otherwise why get into it. Maybe someday I'll write about my views on marriage.
In my experience there have only been a couple of men who I had a relationship with which fidelity would be a part of. One of them was Steve, and I didn't do too well in that department, and worse (for a relationship) I didn't even have any remorse or guilt over it. Ah, I was in my twenties, and a bit more restless than I should have been.
I guess I believe in fidelity as a proponent of a good relationship -- and if it's not part, well that's something you've got to come to terms with your own self.