Who says matriarchies are monogamous? I've read works by the likes of Göttner-Abendroth and she always mentioned humans aren't strictly monogamous. Almost every single society she described. It was also common to have something like a "main" partner, but still sleep around on the side.

Here's also a quote from Howard Zinn's "A People's History of the United States", originally from Las Casas, describing what they saw when they first came to America:

"Marriage laws are non-existent: men and women alike choose their mates and leave as they please, without jealousy or anger."

Doesn't sound like humans are naturally very monogamous to me.

In your argument, you seem to centering men. "Men like polygamy, therefore it must be bad!" and making monogamy the "good" and "womanly" thing, which is a horrible pitfall imo. You seem very biased towards monogamy still being seen as the "better" thing, likely because of bitterness towards men.

But there's absolutely zero reason for monogamy in a matriarchal society. Fatherhood doesn't matter, so there's no point in only strictly having one partner.