In the interest of tribal perpetuation, in stability, marriage was invented. Norms of engagement between the sexes, both within families related immediately in blood and between, had become encoded and practised.
Marriage is the oldest institution since the advent of civilization as we know it, after agricultural settlements by river banks produced culture out of nomads. If behavioral modernity was the result of vocal speech and language, civilization was the result of the end of peripatetic behaviors because the capacity to think had accentuated the possibilities to understand and invent.
The number of men and women in a tribe had determined how many wives and husbands either sex could have. Polygamy was as prevalent as polyandry. The legitimacy of conjugation according to commonly accepted norms was of the essence to maintain social order, not extra-human ordinance. The fear of god or the unknown was perhaps necessary to gather consent rather than the origins of law in a god that was beyond human. God, the perpetual existence of the unknown, was also as much an invention (religion) of the human mind as it has been a discovery (science). Perception is truth. Truth is unknown.
The more explicit understanding of the interdependencies among human beings and nature has also affected marriage. We are now increasingly living in an age when the continued survival of the individual is loosely-coupled from the need to cohabit in a family. And because of it also changes the nature of the family and its relationship with the law.
The law has always been a matter for the state, tribe or nation. When the degree of self-understanding no longer requires the intrusion of the law, the norms of conjugal cohabitation also no longer need to be a matter for the state.
The Constitution of the United States does not include marriage and, therefore, nor should the law. The legitimacy of marriage is, thus, primarily the choice of those cohabiting, while the allocation of resources between individuals (or property rights), cohabitation or no cohabitation, is the domain of the law.
Monogamous or monoandrous, polygamous or polyandrous, marriage has to do only with those in it. People’s personal and social spaces are their own.