Marriage is not like, say, the joint stock company, whose origin in law and history we can date with precision. Marriage is older than the State. Marriage is older than any law defining it. Legislation about marriage does not create the relationship; rather, it recognizes it and attempts to regulate it somewhat. In our legal tradition, marriage was defined in the common law. The common law was derived from judges' opinions in Great Britain, in their need to say what the law they had received from their ancestors was. It was a very, very long time before any legislature attempted to write a statute on it.
Indeed, common law marriage was only abolished in Indiana in 1958. The General Assembly had written statues concerning marriage before that, of course, but had up till that time recognized that an unwritten law from time immemorial could still be appealed to.
The same post on FB maintained that "holy matrimony" was a religious relationship utterly distinct from legal marriage. This is not only shaky legal history, it's bad theology.
Up until around the Reformation, there was no religious marriage ceremony. The rite of marriage slowly evolved from a simple blessing in the church porch to a full-on covenant ceremony. The Church, through its canon law, attempted to define and regulate marriage -- especially within the degrees of consanguinity -- but canon law never replaced common law in England, from which our legal system derives; the two systems existed side by side.
Christians have generally recognized marriage contracted under the ordinary laws of a place as being marriage for the purposes of the Church. Even in Churches which are picky about being married under their sanction (the Roman Catholic Church, for instance), it is recognized that the bride and groom marry each other; they are not married by the officiant. Marriage is what is sometimes called "a natural sacrament." In other words, even religious folk appeal to a definition of marriage which pre-dates religion.
All this doesn't necessarily make the case on same-sex marriage that I would side with, though I find it an important support. My real point here is broader than the issue of the day. Progressives believe that society is made by political processes, such as the passage of laws. They thus believe it can be made anything they want it to be by the passage of a law and by enforcement thereof. And, most important, they believe that they have the wisdom to know what society, what humanity, should be. Conservatives like myself believe that society is a given, that human nature is what it is, and that the best the State can do is try to tidy up the edges. Progressives like to paint themselves as liberators from conservative tyranny; the exact opposite is true. There is no tyranny like progressive tyranny. As both C.S. Lewis and Tom Shippey have pointed out, in different contexts, an old-fashioned tyrant -- a lord in his castle -- may sometimes be satisfied, may sometimes be too tired to inflict his injustice upon you; but a reformer -- someone who is persecuting you for your own good -- will never sleep, never be satisfied, never stop, for he feels that he would fail in his duty if he did not deny himself in order to make you what (he thinks) you ought to be.
At bottom, my objection to same-sex marriage isn't about gay people and their relationships. (As the song says, "You're so vain, you probably think this song is about you.") At bottom, I strenuously object to the redefinition of marriage because the idea that you can simply pass a law or secure a court order or issue an executive decree and remake society is not only foolish, but an invitation to tyranny.