Where do you get the idea that we evolved as a polygamous species? The few remaining hunter-gatherer societies don't work like that. I think that kind of polygamy came with agriculture.
The existence of porn should be enough to show that we aren't monogamous. That and the fact that virtually no other mammal is including all the great apes. Monogamy is a thing in birds and they are literally dinosaurs.
I think possibly we are using different definitions of polygamy. If you mean one male monopolizes the females, I disagree. If you mean that individuals (both male and female) don't mate with just one person, then I agree.
