A reader named Adrian posts a comment in the Huntsman thread that states the situation more succinctly than I have been able to. Emphases are mine:

I’ll give you my perspective as a young person (24) who supports gay marriage. I think there’s a fundamental disconnect between the older generation and this one, and perhaps this might help you to understand it (although I think you already do, to some extent)

Your conception of marriage, the traditional one, is that a man and a woman get married for the purpose of procreation. Marriage isn’t really about romantic love in this conception, but rather a framework for the rearing of children. If we take for granted that this is what marriage is, then I don’t think it’s bigoted at all to not have gay marriage, so long as the coupling is respected.

The problem for people my age is this: your definition of marriage was displaced prior to our lifetime. I have no memory of when that definition was true. Virtually everyone under the age of 30 has lived their entire lives under a culture that believes marriage is an expression of romantic love between two people.

So for a young person with a conservative disposition, the battle against gay marriage isn’t the same as it is for you. You’re trying to conserve something that existed in your lifetime and has since been destroyed. For a young person, there’s nothing to conserve. If the only world they know is one where marriage is an expression of romantic love, any effort to bar a group of people from that doesn’t feel like the conservation of anything, just discrimination.

It reminds me of Japanese soldiers, stranded on far away islands who continued to fight even though the war had been over for years.

My only quibble with this is that older conservatives don’t believe that romantic love has nothing to do with marriage, only that as a social institution, marriage is not primarily about romantic love, but about providing a stable environment in which to raise children.

Nevertheless, it seems to me that the “nothing to conserve” point is at the heart of all this. Young Americans have been formed by a culture that informed them that marriage was about romantic love (and, it should be said, that authenticity demanded the all but unlimited freedom to express one’s desires). Traditionalist arguments make no sense to them, from their first principles.

This is why an ancient tradition — marriage as one man and one woman binding themselves together — endured for centuries, even millennia, but collapsed within 20 years. We, as a people, quit believing the things that supported the older definition of marriage. A couple of years ago, I was at a party at which I heard a young woman in her early 20s talking about planning her upcoming wedding. She said she’s looking for a church in which to get married. Neither she nor her fiancé were churchgoers, and were trying to figure out how to approach a pastor and rent the church for their wedding. It didn’t occur to her that church was anything other than a stage for her dream wedding. From what I could tell, that didn’t occur to any of her young friends, either.

I’m not saying this to criticize her, but rather to illuminate, with this one little anecdote, the vast change in the culture of marriage that has washed over us in the past 50 years. It has been widely observed that marriage in general is becoming a middle-class thing, that the working and lower classes are turning away from it, and increasingly bearing their children out of wedlock. The same shift in values that made gay marriage conceivable, possible, and then inevitable is responsible for the collapse of marriage among that demographic. This is why institutionalizing same-sex marriage makes it a lot harder to rebuild marriage culture among the poor and working classes. Marriage culture is like a river that has changed course, and left we who adhere to the traditional model an oxbow. We are trying to convince others to help us re-channel the river, but that seems not only unlikely, but nonsensical to people who just want to sail on.