A society that does not limit female hypergamy implicitly enables male polygamy, whether it intends to or not, concentrating women around a smaller pool of highly desired men and pushing lower-ranking men out of the mating market.
A society that wants widespread monogamy, men and women of roughly equal rank pairing off, must socially engineer it through marriage norms, reputation, family pressure, and s*xual restraint.
Remove those constraints and nature reasserts itself: women concentrate upward, top men accumulate women, and broad monogamy gives way to informal harems.
The disappearance of enforced monogamy is a hallmark of civilizational decline, a triumph of nature over nurture, instinct over civilization.
Polygamy is the instinct of men.
Hypergamy is the instinct of women.
A woman would rather monopolize the highest-value man she can get in monogamy.
But if she cannot monopolize him, she would rather share the man she truly wants than settle for a man she considers beneath her, or have no man at all.
That is the contradiction:
Her monogamy wants exclusivity.
Her hypergamy wants the best man available.
And when those instincts collide,
hypergamy overpowers monogamy.
Female hypergamy enables male polygamy.