I think you are what you are. People can change, but generally they don't.
My ex cheated on me. After I found out, we separated, then got back together, but after that, I no longer gave a shit and cheated on her. During this whole time I was in the service and we were separated, so it was easy enough for both of us. As is the case with most military marriages, it was doomed from the start, anyway.
Very happy with my current wife. Have never cheated, have never been tempted to cheat. At this point, I've got that Al Bundy philosophy about women anyway - one's enough of a pain in the ass, I certainly don't need two.