I don't think this is true. There is certainly a large degree of individual variation in animal behavior too, it's just not that well studied.

I had a professor who studied owls, which are usually highly monogamous. But there was one male who always tried to have two nests with two different females. It was hard for him because he had to hunt twice as much food, but for whatever reason that was what he felt like doing.

Also we have cultural layers on top.

You don’t see a sheep saving itself for marriage lest it fear eternal torture.

Likewise you don’t see a sheep putting its keys into a bowl at a party.

We are more than our nature.

So you're saying normal humans are incredibly rare. Yep.

Nope, that's not what it means at all

Genetically linked behaviors follow distribution curves. There is always variation in every population for any behavior. When a behavior is highly selected for, alternative behaviors become more rare...but they still happen.

A good example of this is homosexuality, evolutionarily speaking this behavior is highly selected against (reduced babies). It still occurs in all sorts of species.

What this means is that around 2/3rds of humans are strictly monogamous. Around 1/3rd of humans are promiscuous or partially promiscuous. These are instinctual behaviors that can be overcome somewhat by cultural norms. On an individual basis, for every 3 people you meet, one instinctually finds sleeping around to be their "normal" behavior.