Are men capable of having a monogamous relationship?

Not being sarcastic by the way. I am dead serious. Cuz I've seen a lot of guys who have women who are not only drop dead gorgeous but are also completely devoted to their husbands. Yet you still see the husband cheating. Why?. Is it true than men are un capable of being with only one girl?. And please if there are guys out there who are in love with ONLY one woman I'd like to know.