Why do men cheat?

Hello,
After a recent conversation with a friend, we came to a difference of opinions concerning the subject of male cheating. The subject under question is: " If a man is fully satisfied with his current sex partner (in a relationship), is there any chance he would cheat on her ? " Considering that all other spheres of the relationship are ok or great, we've examined the issue purely from the physical side.

Please share your honest opinions on the subject. Thank you !
Cheers,

Alice