[NickM]

This all sounds great as a “just so” story but then you have species like Bonobos that are one of humanity’s closest relatives and they are pretty much the opposite of monogamous. So clearly it’s not naturally inevitable that intelligent species with complex social structures end up with monogamy as the norm.

[s1artibartfast]

Interestingly, due to the promiscuous mating behavior of female bonobos, a male cannot be sure which offspring are his. As a result, the entirety of parental care in bonobos is assumed by the mothers.

It is also worth noting that while they aren't monogamous, Females are still selective, mating primarily with high status males with more to offer.

[nairboon]

Bonobos might be intelligent, but they haven't yet been on the moon as far as i know. You kind of need a bit more intelligence to pull this one off.