It is in the males genes and instincts to mate with as many females as he can to continue his gene pool. If you look in the wild its the alpha male who seeks outs females to mate with so they will bear his superior children to keep a species strong.
So my question is do nice guys finish last.

But does this excuse married men that cheat on their wives?

Peel off the outer layer of a human being back to instincts, after all although a human can problem solve, evolve, feel emotions and make conscious decisions but it is our raw instinct and drive that keeps us as a living species going.
So if a man cheats on his wife is that not just following his instincts to mate with more attractive females. A man will always be on the prowl for good looking females not so much the beauty but to assess her abilities for child bearing even though consciously we don't know it sub-consciously that is what is going on.

On the flip side a female is always looking for a man who has a strong gene and a protective nature suiatble to produce strong children, the traits of an alpha male.