Personally, I have never cheated on my wife of 15 years
I think you have answered your own question!
However, I'll bite. I have been happily married for 29 years next month and in all that time, and even during our courtship, I never once cheated on my wife. When I was at college and also at work the chances to cheat were there. They always will be. The point is, if you truly love your woman and she loves you the biggest betrayal one can do is to cheat on them. I think it says a lot about a person and their relationship if they can walk away from a chance to cheat.
Ian