I'm not disagreeing with your core message: monogamy isn't “older” than polygamy. But neither is it the other way around: We probably did both since very long ago.

The notion that there's a human “tech tree” of civilization is wrong. E.g. Agriculture doesn't “follow” hunting and gathering, and neither does centralized power (like in a state) “follow” agriculture. Humans have been experimenting with social structures since basically the beginning.

So within the last tens of millenia, there were probably societies that were monogamous, some that were polygamous, and some that rotated or did both, and of these some depending on some social stratification and some depending on personal preference.

Source: “The dawn of everything”