See mammals aren't monogamous
Humans are hot
Humans in unions bump into other humans a lot
They'd much prefer screwing instead of the human they've got

from Homeboy Sandman – Strange Planet Lyrics on Rap Genius

Meaning

Despite humans, unlike other mammals, engaging in monogamy and enshrining that in the institution of marriage they often commit adultery and screw around. Homeboy is showing the stupidity of pretending we humans are strictly monogamous creatures

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