Christopher Ryan has a PhD in Psychology. His focus is on the nature of human sexuality. He wrote about it in a book titled, Sex at Dawn: How we Mate; Why we Stray; and What it Means for Modern Relationships.
Ryan makes the case that where sexual behavior is concerned, humans are more like polygamistic chimpanzees and bonobos, than gorillas, gibbons, and Orangutans, in which females generally only have sex with one dominant male.
With chimps and bonobos, females is estrus can mate 1-4 times per hour, with up to 12 different males per day. In bonobo bands, sex is commonly used as a way to maintain harmony within the social structure.
In fact, up until the beginnings of agriculture, the evidence tells us that human hunter-gatherer bands were also polyamorous. Sex used as a way to procreate the species and also as a way to keep the peace.
Ryan suggests that the human monogamistic cultural norm is not only not what comes naturally, it is antithetical to human nature.
The following video from a TED event in 2013, features Christopher Ryan, reporting that human nature is far more tolerant and open to highly social forms of sexual expression than the traditional monogamy that has been the norm for thousands of years.