I've been poking around a bit on this issue of "the churches invented marriage" argument, in the context of the English common law. I've found a nice summary of the state of affairs in the medieval period in 1215 - The Year of Magna Carta
by Danziger & Gillingham.
They indicate that (p. 146): "Marriage had originally been a purely secular affair - one for arrangment between the two families concerned - but it was one that the Church was now [i.e. - ~ 1200 - 1215] increasingly trying to influence." [As an aside, I note that this was a period when the Church was tightening up on another matter of sexual morals, namely the celibacy of the clergy.] A church council held at Westminster in 1200 had recommended that no marriage should take place without public banns read three times, and the exchange of vows in front of a priest. That was to occur at the church door, as a public act, but was not necessarily a religious ceremony. After the couple exchanged vows, they could attend mass inside, but that was not a requirement. The fact that the Council felt it necessary to make this proposal is interpreted as an indication that that was not how marriages were taking place up to that time - that people were going ahead and getting married without any involvement from the church.
And, in fact, that is what the law of the time permitted (1215
, at p. 147). A couple married themselves, by the simple exchange of vows. That exchange could occur anywhere, and did not require the participation of a priest. There were two types of informal marriages, other than a formal exchange of vows in a church: "sponsalia per verba de praesenti" and "sponsalia per verba de futuro". The first was a simple exchange of vows that the couple wished to marry each other, immediately. And that was it, they were married in the eyes of the law. The second was an exchange of promises to marry at some time in the future. That exchange of vows took effect if they had intercourse at any time in the future. From that point on, they were married.
That was the law in England until 1753, when Parliament passed Lord Hardwicke's Act
regulating marriage. It provided that a marriage could only be valid if there were either banns or a licence, and the exchange of vows took place before a Church of England clergyman. (Jews and Quakers were exempted from this latter requirement, but no one else.) That was the state of the law until 1837, when a new Marriage Act
came into force, creating a civil registry
for marriages which churches other than the C. of E. could use. From that point on, marriages in Roman Catholic or non-conformist churches would be valid, as would an exchange of vows before the Local Registrar. It appears from both these links that the pressure for the reforms was as much about creating clear records of marriages and protecting property rights of the wealthy as it was about religious issues.
None of this is to deny the close links between the churches and marriage, but it does illustrate that in England, at least, the church only had a monopoly on marriage from 1753 to 1837, a period of less than a century. Before and after that period, a couple could marry by a simple exchange of vows without any involvement by any church or religious group, although after 1837, that exchange of vows had to be done before the Local Registrar.