I've never been cheated on and I can say with utter confidence that I never will be.
He knows I'd cut his fucking dick off.
My dad cheated for years on my mom before she found out. He had a total separation of love and sex in his mind. It was all one-night stand type stuff, but he'd been doing it the whole time they had been married. She found out when I was 14, so they'd been married 18 years by this time. She didn't leave him. They went to counseling and he changed his ways. They are still together, it's been 32 years now.
When we talked about it a few years ago, she told me that one of the reasons she stayed was she felt too old to start over. She had my brother and I to think of, too. She underwent hypnothereapy to help her get rid of her anger, and her and my dad started acting like teenagers- making out all the time, showering together, etc.
I have to say that although I couldn't make the same decision she did, it seems to have changed their marriage in some good ways. I am also glad that they stayed together and I didn't have to go through the pain of their divorce (selfish as that sounds).
Plus, I believe he knows that was a one time chance. If he ever did it again, well... see my above 'dick' statement.