Where is this mythical Monogamous society you speak of?
There are none.
Did you understand what the study said?
From the older times of 1 to 17, to:
"In more recent history, as a global average, about four or five women reproduced for every one man."
Laws were made to push monogamy, but...
Humans do not reproduce monogamously... the genetic studies do not show it. Genetic statistics show humans to be polygamous.
All of western european, Most eastern european, as well as Chinese and Japanese societies, were all founded on the basis of the monogamous nuclear family structure. The same was the case for all of the overseas colonies that became successful. Of the major religions, only one condones polygamy, and it's precisely the one which is most prevalent in countries that became underdeveloped for many centuries, until recent events turned the tides for only some of them. There would simply be no modern science, technology, and there would only be scarce writing and communication without monogamy. World population would be at most 10% of what it is today.
In the civilized world, everything that is good derives from private property, which relies on monogamous nuclear family structures to function.
Over the past century, and particularly over the past 50 years, the world, and the west in particular, is experiencing a process of de-civilization. That is why you see monogamy being eroded more and more, just as health and everything else is being degraded, polluted, corrupted and destroyed.
Whether paleo people lived monogamously or not is irrelevant, because we don't live in that world anymore. There are good reasons why things changed once we started to develop civilization. The paleo health experience is about undestanding what made them healthy, but a healthy body, and a healthy society, can be very different things at different times.
Today, polygamy only generates war, sickness, poverty, death and despair. With the very real possibility that it could end all human life as we know it. It's one of the main driving forces of the most dangerous thing for humanity. With great power, comes great responsibility. Paleo people were not powerful. Civilized people are. Different times, different needs.