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.