If you look at the animal kingdom you will see being polygamous is the norm. Like gorillas have multiple mating partners. It is simply unnatural for humans to "marry" and only have one mating partner for life. It goes against our innate primal instincts to have multiple mates. The extremely high rate of divorces in America proves my point.
http://www.bbc.com/earth/story/20160213-why-pairing-up-for-life-is-hardly-ever-a-good-idea