I know very little about the history of marriage and family law. I understand Mary Ann Glendon has written a pretty comprehensive book on the subject: The Transformation of Family Law: State, Law, and Family in the United States and Western Europe
I see it's available used for about $10. I might get it.
“Legal” marriage is always defined by the government or authority, and always has been, and in all societies.
Rome, Greece, Europe after the Catholic church gained the power to be the authority deciding what was legal marriage, Indian tribes, Islam, they all have to define legal marriage.
In America if you don’t care if your marriage is legal or not, then don’t bother with the law, many Mormons and atheists, and Muslims, already do that.
You can also get legally married in some states, without a license, but you still have to be able to prove that it is legal, IF, you want it to be legal.