Men as not genetically made to be fathers in monogomous relationships in the way tradition implies. Like most members of the animal kingdom, a male's primary purpose is to spread as much of their gooey genetic seed around the world as they can. The rest just comes down to territorial actions that always lead back to trying to establish higher standings in the pack - which of course leads to more female pun tang.

One could take a different approach and say that this cultural shift is a result of men breaking away from the religious based stereotype of being a provider and more towards a true male existence.