Sacred to harmony,
Sacred to love.
Newburgh Hamilton, libretto to Alexander's Feast by George Frideric Handel
"The sanctity of marriage" is a phrase which causes many problems (or solves them, depending on which side you are on) when introduced into political discourse. This is due to both the form of the phrase--a conjunction of two abstract nouns--and the manifold ambiguity of both terms.
Starting with marriage, it can mean either a religious ceremony held within a place of worship, or a legal procedure regulated by the law of the land. Which is the speaker of the phrase referring to? Or are they (intentionally or otherwise) creating an unresolved ambiguity: fudging or weaseling? In addition, there is the minor ambiguity between the act of marrying or becoming married, and the state of marriage as enduring over time.
But "sanctity" is the real verbal villain. It means "sacredness", this much is clear. But in what sense? Holiness? Blessedness? Venerability? Religiosity? Devotedness? Consecratedness? The state of being dedicated to a single use, purpose or person?
This last meaning--set aside for a particular use, purpose or person--deserves more attention. It's the same as in "Is nothing sacred any more?" The essence of this form of sanctity is that not just anyone is allowed in, and you can't just do anything you feel like. The most sacred is by definition the sanctum sanctorum, the inner sanctum to which only the blessed few are admitted. More generally, sanctity means that there are rules to protect and limit the use of something. Although the ground inside a church is no less and no more part of God's creation than any other piece of land, the Church and society have instituted rules prohibiting many types of behaviour inside a church. And it's not just religion: you can have a place sacred to the memory of a person; or a concert hall devoted to a particular type of music; or a toilet dedicated to the use of a few privileged individuals. They all have their own type of sanctity, one very much of human, rather than divine, origin.
So "sanctity of marriage" can mean the religious and venerable quality of a certain church ceremony, or the situation in which the state of marriage is subject to law and you can't just do anything you feel like; or, indeed, something else entirely, depending on which of the myriad connotations are in the speaker's mind. Now, I don't see anything particularly wrong with either of the two definite meanings I've just outlined: many churches' marriage ceremonies are indeed religious and venerable, and if you are a co-religionist you may well think they are also holy and blessed. Conversely, the legal state of marriage is not one that can or should be conferred just anyhow on anyone: it's probably a good thing that a person cannot be legally married to a dog, that a 13-year-old cannot marry, that one cannot be married to six people at the same time.
So what is the problem? When someone is trying to have it both ways, or even several ways at once. The Weasel Triple Crown goes to the statement "I believe in the sanctity of marriage", since "to believe in X" can mean both "to believe that X exists" and "to believe that X is good and effective". So the sentence can means "I believe that marriage is holy, venerable and blessed", or "It is a good thing that marriage is restricted to certain uses, persons and purposes, and not extended to others". And, inevitably, in the minds of many listeners, there will be an implied connection: Marriage is a holy, blessed institution of religion, thus it is a good thing that it should be restricted to certain persons and purposes. Without explicitly saying so, the phrase imports the idea that religion is to play a definite role in informing the law of the land.
The ability of a few short words to encapsulate by stealth a flawed, but seductive, argument makes a breathtakingly effective political slogan. The unusual, talismanic nature of the word sanctity helps: it is never used in ordinary conversation, so people are unused to unpacking its meaning(s). And the use of an abstract noun phrase (rather than, say, "I believe that marriage is sacred") tends to reify the whole complex of ideas: the repeated use of the phrase builds up the notion that "the sanctity of marriage" is a real thing with an existence of its own, not just some guy's opinion. The same process can be observed with fair trade and Axis of Evil.
So what do I think about marriage? Well, unlike certain speakers on the subject, I can make it clear that I'm leaving out any consideration of religion, because as we all know Congress shall make no law respecting an establishment of religion. And every religion has a slightly different (or very different) view of marriage. We're left with the civil, legal aspects of marriage, whose "sanctity" consists precisely in the rules and restrictions that are placed upon it.
Now marriage is simply the most effective formal method of regulating interpersonal relations that society has ever discovered. Some people, anarcho-libertarians among them, would have it that people can get along just fine without formal means of regulation imposed by the state, relying instead on contracts drawn up to order. But marriage as we know it would not survive, since it is an implicit contract between two people and the state, including things such as taxation, inheritance, hospital visiting rights, etc. So as long as there is a state, there will be marriage.
A good marriage is a good thing, and a bad marriage is a bad thing. The object of the rules and restrictions that "sanctify" marriage in law should obviously be to create more of the one and fewer of the other, while respecting the fundamental principles of law. This is the only valid argument for changing the law: to appeal to any other form of sanctity is to make a fetish out of something that has no business appearing in a discussion of law. The business of the marriage law is to make good marriages, not to advance religion or religious attitudes, nor to promote the mysterious and eternal power of love between two people, nor to produce and rear children, either well- or ill-behaved or -nourished. Once the law is made, we have no business complaining if people turn it to their own uses, except so far as this is evidence that the law needs to be changed.
With regard to gay marriage, the Founding Fathers have more or less already told us what the answer should be. But concerning divorce, age of consent, alimony, and all sorts of other issues, it's more than conceivable that the current legal state of marriage is in need of emendation. I'm open to any reasonable arguments, as long as they don't involve the word "sanctity".