the grey areas of cheating

[Dawn]

Just thought I'd point out that Polygamy is also gender unspecific as is Polyamory. According to the wikipedia article on Polygamy, the word you were looking for (a man with many wives) is Polygyny.
Polyamory: A person with multiple sexual partners.
Polygamy: A person with multiple spouses.

Polygamy typically refers to a man with multiple wives.

Therefore, it's sort of a misnomer to use polygamy when polyamory is a flat out better word.

That and a spouse is a marriage term. Polygamy in America is impossible, because even if you cheat, you're not married to the person you're cheating with.

Notice how you said the inability to be monogamous is a result of society. Nature isn't intending for us to do something stupid, it's our societal/cultural norms that cause that problem. You're trying to reconcile societal/cultural responsibilities with our biological tendencies, and they don't mesh very well. Remember, marriage as we see it is a relatively modern concept. Our ancestors in Neolithic times didn't have such concepts.

Why does cheating continue to happen then, even in our modern culture? Seeing as Polyamory has existed far longer than any cultural concepts of marriage, it would hint at a biological factor.

...You just managed to take what I said, snap it's spine, and twist it until it was backwards. Humans have monogamous traits and polyamorous traits. They both have logical roots. Neither is absolute in the human race. I'm basically saying the same thing I said before, except with less room for twisting.