Update (2008-11-20): In this Slate.com blog entry, professor Doug Kmeic of Pepperdine University proposes the same thing!
In the debate that has followed the approval of Proposition 8 in California's November 4th, 2008 election, it occurred to me that this entire controversy could have been avoided had organized religions not allowed a sacred ceremony (the 'wedding') to fall under the jurisdiction of government.
Acknowledging the fact that Wikipedia is not necessarily a reliable source, but realizing that I have nothing better readily available: It appears that marriage was first codified into law somewhere in the early 1500's under the influence of John Calvin and the Protestant movement. Prior to that time, and certainly throughout most early written history, the institution of marriage and the ceremony that preceded it had been purely a non-governmental matter. Understandably, the leaders of that ere never foresaw the need for a formal separation between religion and the state.
But alas, the legacy of that decision is what we are left with today.
In keeping with the general principle of religious freedom, organized religions should be free to define marriage in their own terms. They should be able to enforce any requirements and qualifications they deem necessary on those persons who wish to undergo a wedding ceremony.
At the same time, governments need a working definition for 'people who willingly join together for the purposes of domestic organization and/or the rearing of children'. The result of which is that governments are now put in a position of creating the legal definition for what was once a private or religious agreement. The result? Acrimonious political battles that will likely continue.
Besides, anyone who has been married can tell you that it is not always civil (see definition #5).