I'll bet he can't answer the question about why he did it any more that any one of us can answer the question as to why we masturbate. It's what humans do. No, I never cheated on my wife. One of the big factors as to who has affairs and who doesn't depends a lot on opportunity. It's a lot easier to be monogamous if you're fat and ugly or too old to get it up.
Most people though have opportunities which they either resist or give in to.
The more opportunities you have, the more likely you are to give in. Imagine what it's like to be Bill Clinton. Women are attracted to power. In every species of animal the females want to mate with the strong and powerful males. Clinton is the most powerful male on the planet.