Every animal species has a social structure that they base their daily lives on .. and I don't see why humans would need to "spread around" .. we don't live in packs or herds we live in families .. father mother, brother, son, daughter ..
It's been like that forever .. and even in the deepest untouched parts of Africa (like I previously stated) where "man" has not interveened and put "morals" into play .. the societies that have been foiund .. live almost the EXACT same way. They choose a wife/husband and start a family .. the man doesn't just run aroiund poking his dick in everything that will let him, and the women don't encourage it.
Watch Discovery Channel some time when they do documentaries on those people.
I don't see how one can say "humans" are meant to not be monogamous, when it's clear that even when not taught the "morals" that we civilized people operate under .. that they still bascically do (in a round about manner).