Those are some cute theories, Sociopath, but I can't say I've found them to be true in my experience.
For instance, I'm a woman who did cheat on her handsome, supportive, sexually skilled husband. Why? Just because I wanted variety. I wasn't moving to greener pastures. I just wanted to mix it up!
I sort of feel like most people have the urge for variety but then suppress it if they're trying to be monogamous. And that's all there is to it.