The discussion seems to have gone off into some interesting directions (biology, archaeology, anthropology), but that is beside the point. The idea of the state existing separate from the church pretty much started with the US Constitution, so going back before that is irrelevant to this particular discussion.
The issue is not about church-sanctioned marriage, it is about why US marriage laws exclude same-sex or multiple-partner marriages. If anyone can show a good, non-religious reason to limit marriage to one-man-one-woman, I’ve never heard it (and I did start a thread on this a couple weeks ago and did not hear any good argument there). The framers of the Constitution did not mention marriage, but they did refer to making laws respecting a particular religion.
As it has been stated, marriage contracts are about property and children. But no-one forbids you to marry if you are sterile, or unlikley to own a lot of stuff, or are simply disinclined to have things. Besides, inheritance and pre-nup contract take care of most of the property issues, and there are tons of laws on the books protecting children’s financial well-being, regardless of the marital status of it’s parents.
Ask people why they got married and they will only rarely mention children and property, but they will wax rhapsodic about love, companionship, friendship, sex, etc. Sometimes wanting to have children or security will also be mentioned, but usually only in conjunction with the emotional reasons. So, since that the reasons people marry has changed so dramatically in the last couple hundred years, why hasn’t the state updated the laws, or more correctly, why haven’t the challenges to these laws succeeded in opening up marriage options beyond on-man-one-woman? I can see no non-religious reason why anyone who wants to marry (assuming they are consenting adults) should be excluded from having a legal secular marriage.
(And btw, I don’t believe that humans are serially monogamous. If some of them occasionally act like they are it is usually because they are using intellectual filters to overcome biological desires. Especially true of human males, but not entirely exclusive to them. We would hardly need so many laws controlling natural forms of behavior if there were no reason to suppress them. And can you really imagine that the world’s oldest profession would be so universal if we were monogamous animals?)