Marriage is and always has been about rights in property. When most of the population were serfs, or even free men and women who had no free hold, they were far more likely just to live together and rear their children without benefit of clergy. Even as late as the Edwardian era, this was true. Robert Roberts, in what is now in its own right a classic--The Classic Slum: Salford in the First Quarter of the Century
--says that fully three quarters of couples rearing children had never married. This is the slum which Friedrich Engels described in Conditions of the Working Class
Essentially, no one cared how people without property lived their lives. The United States became a different case because there was land enough for everyone, and most immigrants after the revolution came precisely because they could have land, which they couldn't in Old Europe. This was even so for convicts in the era before the revolution. Thomas "Stonewall" Jackson was descended from a pair of convicts who met in England, and married in Maryland after they had earned their tickets of leave. By the time Thomas and his sister were born, the family was enormously prosperous, and the "first family" of their western Virginia county.
Property ownership makes all the difference. The religious right tries to make it an issue of a sacramental nature, but marriage has always been about rights in property. I don't deny that married couples often love one another, or that they marry for love. But the institution
is concerned with rights in property.