Earlier quoted context omitted.
Is the U.S. moving away from monogamy? I'm not against polygamy or polyandry, but I don't see a trend in the near term. On the other hand, we've long had a tradition of traditional married couples where the man keeps a mistress on the side, usually in secret (though secrets don't tend to last very long).
No exactly the same, but it is becoming more common for women in the United States to have children to multiple different men in quick succession. Definitely a form of polygamy. Particularly of note in poverty stricken communities and the African American community which also happen to have many social problems. Its an interesting theory worth looking more into IMO.
While sometimes people refer to "serial polygamy", and there are some contexts where its actually reasonable to view that as a kind of polygamy, here, where the focus is on formal, recognized marriage structures and whether men have the opportunity to enter into them, it isn't the kind of polygamy that is at issue, both:
(1) because its not a formal, recognized marriage structure, and (2) because even if it were, it would be polyandrous, and the paper is comparing polygyny (not "polygamy" more generally) to monogamy, so this would be driving past monogamy on to the opposite side, if it were relevant at all.