Daniel A. Crane, Associate Professor of Law at Cardozo School of Law offers an excellent essay on A "Judeo-Christian" Argument for Privatizing Marriage HERE.
As do I, Professor Crane realizes Civil Marriage as a completely separate institution than Religious Marriage and gives a very detailed explanation on the dangers involved with merging the two. He further believes that the Christian Right's attempts to legally define marriage through constitutional amendment will do far more to ruin the religious concept of marriage in the minds of people as well as the legal grounds for religious freedom, than it will do good.
At the onset of his essay, he gives a pretty lengthy history of marriage in both the Jewish and the Christian traditions including how, in the Jewish tradition, civil marriage is still viewed as irrelevant in contrast to the mainstream Christian practice of placing marriage, by definition and validity, in the hands of modern government.
Even if you disagree with thoughts of marriage being a private, religious institution ONLY, I think you'll find Prof. Crane's claims to be rather thought-provoking and a worthy read.
I'm just happy to not be alone in my opinion of legal marriage being TOTALLY different from biblical marriage (and did you know that C.S. Lewis had similar thoughts?). :)
Happy reading and Shabbat Shalom to all!
P.S. Here's a cleaner PDF version if you care to print it out:
A Judeo-Christian Argument For Privatizing Marriage by Daniel A. Crane