Just 16 years after a Democratic president signed the fatuously named Defense of Marriage Act, defining marriage in the United States as requiring one man and one woman, the debate over gay marriage is over.

Isn't it? Even though DOMA is still on the books, even though most states that have voted on the issue have voted against same-sex marriage, all the energy is in the opposite direction.

What seemed at first like a bizarre idea has become utterly conventional. By judicial decree interpreting the state constitution, by act of the legislature and someday soon by popular referendum, one state after another is falling. Same-sex marriage is legal in Canada.

Does anybody believe that five years from now it will be harder than it is today for two women or two men to marry? It's no longer all that hard today. I suspect - don't you? - that even many anti's have given up in their hearts and have resigned themselves to taking comfort in one more example of how the country is going to hell.

"What's next?" opponents of same-sex marriage have sometimes asked, they thought rhetorically. If a man can marry a man, what about a man marrying two men? Or two mixed-sex couples merging into a married foursome? Or - the inevitable reductio ad absurdum - why shouldn't a man marry his German shepherd if he wants to?

No doubt these opponents enjoyed (and deserved, actually) a warm I-told-you-so moment over a recent headline in the New York Times: "Measure Opens Door to Three Parents, or Four." It was about a bill in the California Legislature - California! Home of the famous Proposition 8, a successful ballot initiative to outlaw same-sex marriage - to allow adoptions by more than two parents.

The bill is about parenting, not about sex. (Even in California, there are only two sexes, approximately.) But the bill recognizes the reality of unconventional families: divorced dads who want to keep a close relationship with their kids, lesbian couples who want to adopt each other's children, and so on.

And the opponents of gay marriage are right. Once that initial wall is breached, a lot of this suddenly seems to make perfect sense. Where they're wrong is to think that this is a good argument against same-sex marriage.

Every big societal change carries more change in its wake. And every change is a revolution in perceptions. From the present, you look back 20 years and think, "Why did we find the idea of same-sex marriage so weird?"

Twenty years from now, gay marriage will be so common that people might be forgiven for thinking that the Defense of Marriage Act was passed to protect gay marriage.

This is one good reason for reserving some sympathy for those who aren't wholly onboard as the train of change comes whistling through: There is something you think today that will seem preposterous and even offensive to your 20-years-from-now self, if you're still around. Some injustice that will seem obvious, although right now we can't see it at all. What will it be? It would be nice to get a heads up.

"Not every disputed institution or practice is destined to be discredited," the Princeton philosopher Anthony Appiah wrote a couple of years ago. Looking back, he contrasted abolition (a cause that came "to represent moral common sense") with Prohibition (a cause eventually seen as "quaint or misguided.")

Appiah suggested three signs of a practice that seems harmless today but will seem indefensible tomorrow (or, presumably, vice versa). First, "a particular practice is destined for future condemnation" if the argument against it has been building for a while. "The case against slavery didn't emerge in a blinding moment of moral clarity."

Second, the defenders of current practice "invoke tradition, human nature or necessity" rather than morality. Third, the defenders engage in "strategic ignorance." We might say they are in denial about "the evils in which they're complicit."

Today's prohibitionists and abolitionists are already working on some issue that will look completely different to most of us two decades from now. What is it? Appiah has four nominees:

-- Prisons. We incarcerate more of our population than any country in the world. Jokes about prison rape are staples of American comedy. In 20 years, we may look back in amazement that people would think this was funny.

-- Industrial farming. The longstanding discussion of the conditions under which animals are grown for food is turning into a discussion of the morality of using other animals for food at all.

-- The elderly. Baby boomers already feel guilty about how their parents spend their last years. Just wait until it's the boomers' turn.

-- Greenery. Environmental degradation is a debt to our children that parallels the debt to our parents.

My own favorite nominee will win me no friends: high school football. In 20 years I think it may seem incredible that loving parents used to send their kids out to bang their heads against one another in the certain knowledge that this was damaging their still-growing brains. "Certain knowledge" may overstate the case now. But this smells just like smoking, about which the evidence dribbled in until it was undeniable. Let me add (for my own self-protection): I hope I'm wrong.

Suggestions from colleagues ranged from weighty moral issues to relatively trivial rules of grammar. In two decades, will it seem incredible that people in 2012 were generally unconcerned about the military use of drones? That we didn't have a national identity card? That Americans regularly and unapologetically used "which" when we should have used "that" (or vice-versa)?

One colleague suggested "free wireless." Does he mean that we will be amazed that anybody paid for an Internet connection as recently as 2012, or that anybody thought they could have it for free?

Same-sex marriage is not yet universally accepted. But it's not too soon to start looking for the next sea change. What will it be? Post your nominations to Twitter using the hashtag .headsup.