Biological evolution brought us to a certain point, but when humans evolved to the point where fewer people died before having children it ceased to have quite the same potency. For biological evolution to be effective, defects must result in the death of an organism before they have a chance to mate.
During the last 50–100,000 years, evolution of humans has been on a cognitive level. Sure, our distant ancestors had biological reasons to copulate as often as possible, but we gain greater advantage through our intellectual advancement. It is no longer necessary, or particularly advantageous, for a man to sleep with multiple women, and so that desire has become muted in favour of our new found need for the intellectual fulfilment of a good relationship. We now value loyalty over pure production.
This argument is a falsehood, used by bigamists who are insecure with their personal want for multiple partners, and so extrapolate their ideas to the general population.