Well, here’s the Wikipedia. To be clear, I’m counting a society where elite men might have multiple wives as still monogamous, since that’s not representative of an average member of the population and the wives themselves are still bound to a single partner. Maybe that’s a terminology error but for the sake of this question I think it’s clearest.
And yeah, as someone pointed out there’s an amount of infidelity in every human society, but it’s generally neither endorsed by the legitimate partner or society at large, at least not as an actual relationship.