Human civilization, a while back, decided it was better if everyone had "skin in the Game", rather than Alphas getting all the poonani, and so this concept of monogamy came about. This was purely to stablize society and make men not so sex-focused. In case you haven't noticed, men are obsessed with sex to the point of being dysfunctional and an utter detriment to society.
Up until recently the rules were this - 1 woman for 1 man. Being a player, cheating etc was all shamed. The effect this had was that everyone was getting laid, and everyone could focus on other stuff because their sexual needs were being met (largely). The only people who didn't like this arrangement were Alphas and Women. Who was behind feminism? Alphas and women.
Those days of monogamy and fairness are over now. We are back in a free sexual marketplace where alpha's get all the sex. What's weird though is we still like to pretend that we are living in a monogamous, stability-focused and family-focused society - and pine for those things and wonder why we can't have it. It's because we are back living in the jungle again.