Prove that men and women weren't meant to be monogamous? Or do you believe we're just evolutionary automatons who are slaves to our genitals? Are you a single male?
Men are biologically driven to "spread their seed" as much as possible, which is why polygamy was the norm, rather than the exception, in most societies.
There's empirical medical evidence that men and women are meant to be monogamous. Women who have their children by one man have less health problems with the pregnancies, have healthier children, and have less death in the years that follow. Men and women are both healthier when they are monogamous, less STD's at least. Married men live longer than unmarried men.