Proof? I'm a man, and I'm telling you it is true. We men know these things. Do you want to talk to a list of my guy friends that are either cheating, or on the verge of doing so because of this very reason? I'm not condoning it, just explaining it. Trust me it is true. I'm not saying that this is the only single reason that men will cheat. I'm saying that it is a big reason.
There are certain maxims that exist with the creature known as the human male. This is one. I didn't make it up, I didn't create man to be this way, I don't even always like being a man, I'm just telling you that that is how things are.