In biological terms, the male who impregnates the most females wins. He ensures the continuation of his species as well as his own DNA. Me thinks this is something that's hardwired in the male of most mammals -- perhaps even most species.
Not all men cheat just because. I would not hesitate to cheat on my wife, not because it is something I want. We have been living as basically roommates for a while and I want to be with someone who desires me. The only reason I have not gotten a divorce is because our kids need to be provided for.