Polygamy and slavery have been going on since the dawn of time also.... they are just as valid as your point ( and blow a hole in your invented pair bonding nonsense)
"Valid" in what way? They have nothing to do with each other, nothing to do with pair-bonding, and little to do with marriage. So why you brought them us is a mystery.
Human beings are by default not monogamous….
That's not true. We are generally monogamous, just not generally for an entire lifetime. We pair-bond for long periods of time, usually based on a set of conditions, but eventually those conditions change, and the bond dissolves.
... the divorce rates and rampant cheating show that.
Is cheating "rampant"? I don't think it is, really. Most pair-bonds remain monogamous for most of the length of the bond. But that's not the length of one's life, in many cases.
We need to get out of the marriage business..... all it does is cause suffering and poverty when the majority of them fail.
Who's "we"? Humans will pair-bond. That's a fact. They always have and they probably always will. And as with all interactions between human beings, people need to be protected from each other. So like it or not, the government will have to protect people from their mates, on occasion. And that requires laws and oversight. There is no escaping these simple facts of reality.
The Quakers were on to something.... they never got licenses and never had ceremonies... their marriages were by consent of the congregation.
And their communities have laws and consequences for breaking those laws, just like any other society does. The only difference is that they are religious laws instead of secular laws.
If you want to live in a theocracy move to Iran. They'll be happy to give you a set of religious laws to follow, and make you pay the consequences if your don't.