The threat to American monogamy is not polygamy but hypergamy. That means marriage into an equal or higher caste or social group, also known as "marrying up". Evolutionary psychologists say that women have an instinctual need to mate with higher status men.
The past half-century has seen many trends contributing to the destruction of the American family. Feminism. No-fault (unilateral) divorce. Cheap and reliable contraceptives. Welfare queens. Social acceptance of loose women and single moms. Sandra Fluke lobbied for free contraceptive pills for law students, and even got a telephone call of encouragement from President Obama.
Underlying all these trends has been unleashing female hypergamy. It no longer just means marrying up; it means hooking up with alpha males. Women are attracted to wealth, status, body type, and personality, as always, but they no longer need to marry to get what they want. And the alpha males don't need to marry them either.
The alpha males who enjoy indulging female hypergamy call it "game". I am not sure if game is short for hypergamy, or if the term comes from treating seduction like a game with goals, rules, and tactics. Either way, feminists are now liberated to pursue their hypergamous ideals as long as they retain their youth and beauty, and no one calls them sluts anymore.
Some of these trends are unstoppable. But some of them are byproducts of govt policies promoted by feminists and liberals who are out to destroy the American family. Unfortunately, there is no political will to make the necessary changes. See, for examples, Roissy's plan to save marriage or his anti-alimony rant or anything else he has to say about social trends and hypergamy. His posts are outrageous and offensive, but they are also grounded in human nature and scientific research. No one is going to pay any attention to him until current social trends get a whole lot worse.