More on the gold and sandstone analogy. My son John writes:

You wrote: "Gold is, obviously, not like marriage, because people have an interest in accumulating quantities of gold, but each person can only have one other person in the marriage market."

Well, let's assume for the sake of argument that we are interested in viewing marriage in terms of possibly "devaluing marriage" in some kind of a quantitative way. (I don't agree with this quantitative way of looking at marriage, but if that's what people like Banuchi want to do, then they should be argued with on those terms.) In that case, not only you are you right that gay marriage wouldn't seem to devalue marriage very much, but couldn't we also say that divorce devalues marriage a lot more? 50% of marriages end in divorce, so I assume that the number of divorcees is much larger than the number of gay married people that there would be if gay marriage were legal. Now, let's assume for the sake of argument that divorce and gay marriage are equally severe breaches of Traditional Morality. We would then need to say that divorce threatens marriage a lot more than gay marriage would. You said that "each person can only have one other person in the marriage market"--well, this doesn't quite apply to divorce, since one person can get divorced many times. If someone gets divorced five times, then that person has chipped away at the institution of marriage five times; this is in contrast to the gay person who gets married once, thereby chipping away at marriage only once.

(Maybe gays would be more likely to get divorced, because they're radicals who want to erode traditional institutions. Or maybe they'd be less likely to get divorced: maybe they'd value marriage more, in the way that someone living in the desert for years might go on to value water more than other people do. This is an empirical question that we don't yet have the data to answer.)

...So, anyway, my point is that the true conservative who genuinely wants to speak in quantitative terms about preserving the puritanical monolith called Marriage should be focusing on the current horrors of divorce instead of the potential dangers of gay marriage. (None of this matters anyway, since marriage doesn't work like gold, for the reasons you pointed out. But the conservatives should be argued with on their own terms.)

Great points. Let me add that anyone who invoke the Traditional-Morality-Attack-On-Marriage argument has a real problem if they say it's okay to make an exception in the case of divorce but not okay to make another exception. You can't rest very solidly on a foundational principle if you tolerate exceptions for the things you and the people you identify with might want to do and then refuse to make exceptions for the things that only other people might want to do. That is the most unprincipled thing of all. Better to be completely flexible than to stand on inflexible principle only when it serves your own purposes.