Are animals monogamous?
Marriage requires a special talent, like acting. Monogamy requires genius. –Warren Beatty
Monogamous animals are uncommon, but monogamous mammals (like humans) are especially rare. When monogamy IS observed in the animal kingdom, it’s only seen in species that require that both adults support their offspring. So, given the huge amount of resources needed to raise children to adulthood, the (MOSTLY) monogamous behavior of humans is unsurprising.