In a way, Americans have already moved past being a truly monogamy-based society, especially in comparison to social (and political) norms for the country's first century. The compulsory expectation that you will enter marriage with one partner for your life has significantly shifted in the last 50-100 years.
Outside of extremely religious communities, it is simply not demanded that you only sleep with one partner and remain with them for the rest of your life. Hookup culture, especially among college-aged people, has become the new norm. Four in 10 children are born to unwed mothers. Divorce and remarriage is common. Around 30% of husbands and 20% of wives have had extramarital affairs. About 4%-9% of Americans are currently in open marriages (for comparison, LGBT people make up only about 4% of the US population).
Monogamy is what we say we expect, but we don't actually police sexual relationships in a way that enforces that as a social structure anymore.