This can be nature or nurture. By nature, I mean that this mentality is used as a defense mechanism. He doesn't want to be thought of as weak, so he does something that makes him feel strong. Cheating is one way he can achieve this and abuse can be another. Some people use drugs or alcohol for the same affect. Perhaps the reason that cheating is so common is because it's one of the few options that isn't illegal.
When I say nurture, I mean that his whole life, he has been taught by people close to him or by society that doing this is what makes you a man. "No pu**y like new pu**y," they claim, yet they want to say that it's all the same.
When a woman does it, the reasons that I notice most are either because of insecurity or independence. Having sex with many men can help make some women feel desired. For other women, cheating can help them establish the fact that they don't need a man. I've never seen "the playa mentality" manifest itself positively, but never in history has it been so glorified.
Some would say that having side relationships will help you stay with the person you're with for longer but if the benefits are not on both sides than it benefits no one, not even the person or people who think they're benefitting.