That's nonsense. I'd stay married to my wife even if it were illegal.
For example: different taxation, different financial aid, etc.
You could get rid of all these, but that is a big change.
I don't think that's the full story. I suspect that religious marriage existed long before nation states and most of the current legal distinctions associated with marriage.
I suspect that marriage - a man/wife pairing (or 1-to-N grouping) recognized by the tribe/village with respect to all those legal distinctions related to property, children, etc... - existed long before the religion.
Do you have any actual evidence? Because here in reality-land, the evidence shows that the English legal tradition considered marriage a civil institution long before the churches tried to claim it.
Hardly the place to look for the origin of ancient traditions...
Wat.
The simple fact is marriage easily predates all the English (pre or not) legal traditions. English (pre or not) traditions at best drew on the earlier concepts and practice of marriage.
Crack a world history book open sometime. Or just use google.
For example: http://www.amazon.com/Anxious-Pleasures-Sexual-Amazonian-Peo...
And historically in the West, marriage as a sacrement didn't get off the ground until the late Middle Ages: the Council of Verona in 1184 if memory serves. And even long after that it was still mostly a practical matter for most people.
As to the legal situation many have mentioned, that could have easily been covered by a will for us. It may vary by country, but a will and defacto status offer the same protection in some parts of the world. I sought legal advice when I considered my options of marriage because marriage means little to me. I think its significance to a couple's commitment is highly over rated.