Perhaps the title of the thread is ambiguous for most of us. It generalises that men are always unfaihtful to their partners.
For someone like me the experience is arc-opposite. All men around me in my family, friends and social circle are dedicated, loyal husbands or boyfriends. In fact the first person, divorced, I met was a British woman, who admitted that she met her current boyfriend while she was married and then left her huband.
Also we must understand that biologically every female organism have distinct phase in her life when she concentrates more to the upbringing of her youngs, in which she undermines companionship. So such phase motivates the male to search for a new female.
I also agree that being intellectual and human, men cannot use this fact as a reason and start being unfaithful to their partners. But men are more sexually motivated than women. So when they find a void in physcial satisfaction they tend to get unfaithful.
And mind you, Sex/Money/Power are powerful motivations. Even women cannot escape the temptation.