I don't say monogamy is a natural rule everybody should follow but it also is not unnatural "bullshit" invented by humans. As far as I know many wild species are monogamous. Many bird species in particular. I don't mind anybody being polyamory (and even some sort of adore them for breaking a taboo) yet many people are happy in monogamy and they are not necessarily constraining themselves artificially. Also shame can hardly be considered "invented" - whenever you feel that blushing you don't invent it, it's a physical body reaction although particular reason behind usually is "invented".