Republican politicians’ reactions to the Supreme Court’s ruling on same-sex marriage went a long way toward explaining why their side lost. Their press releases were generally defensive, palpably eager to change the subject, a testament to the fact that most Americans now support same-sex marriage, judging from most polls over the last few years. Even more telling, though, was the nearly uniform failure of the Republicans to state a rationale for rejecting same-sex marriage.

They said that the Court should have left this matter for the people to decide, and that they themselves believe marriage is the union of a man and a woman. To the extent that they offered a justification at all, they said their faith entails this view. That invocation of faith was itself a defensive move: Don’t blame me, blame my church. But it was also in keeping with the pattern of the debate over same-sex marriage even when it was unpopular: Opposing politicians rested their case on their faith, on tradition, and on majority sentiment.

“I believe that marriage is the union of a man and a woman. Now, for me as a Christian — for me — for me as a Christian, it is also a sacred union. God’s in the mix.” Asserting a position, and then invoking faith: That was Barack Obama in 2008; but it was not much different from the way politicians who actually believed what they were saying talked.

The invocation of faith was not compelling to people who do not share that faith, and it raised — still raises, for the Republicans who still lean on this rhetoric — the uncomfortable question of how they decide which aspects of their faiths they want public policy to reflect. Majority support for traditional marriage was not an obstacle for people who concluded that the tradition and majority view should change.

Sherif Girgis, Ryan Anderson, and Robert P. George put forth an actual argument for the view that marriage law should concern the union of a man and a woman. The argument was that marriage originated, both as a social institution and as a legal category, to deal with the facts that sex between a man and a woman often generates children and that these children are most likely to flourish when their father and mother are united for the long run. Marriage exists, that is, to regulate the behavior that can produce children, to channel that behavior toward the best interests of children.

That argument was not made up to exclude gays and lesbians from marriage. Something very like it lay beneath family law long before anyone had the idea that two women should be able to marry. It’s worth noting that the argument is not that same-sex marriage is impermissible because gay sex is wrong. If anything, the related traditionalist argument would run the other way around: Gay sex is wrong because it can’t be marital.

The social-conservative argument is that same-sex marriage is both wrong in principle and likely to have negative consequences of uncertain magnitude. It would teach people that marriage was primarily about the emotional needs of adults; it would make it harder to argue the case that children need a mother and father committed to them and to each other. The norms linking sex, marriage, and procreation, already weakened over the last several decades, would grow weaker still.

This argument had only a spectral presence in the popular debate over same-sex marriage. Politicians were generally unfamiliar with it and unwilling to make it. In these respects they represented the broader public, which no longer agreed with or understood the premises underlying the old norms. That’s why I argued in these pages twelve years ago that same-sex marriage was likely to prevail, notwithstanding the polls that showed that most people disagreed with it. The fundamental concern of same-sex-marriage opponents was that the public would lose the old understanding of marriage. But that battle had largely been lost before the debate over same-sex marriage even began.

The public policy of marriage did not aim to promote either liberty or the general welfare directly, but only by shaping social morality. When the debate over same-sex marriage began, we hardly even had a vocabulary to discuss such a policy. One of the most potent arguments for same-sex marriage was that it “would not hurt you,” or anyone else’s marriage. The implication was that people opposed it more for its symbolism than for its concrete effects — something that could just as rightly be said of its supporters.

It was also a libertarian, you-can-swing-your-fist-until-it-hits-my-nose way of talking about the issue. But such terms do not fit marriage well, because the logic of libertarianism tends to cut against the state’s recognizing marriage at all. (As does the logic of same-sex marriage: Why would the government have an interest in romance as such?)

All that was left was for the public to begin to see homosexuality in terms of identity rather than behavior. Once that happened, insisting on the old definition of marriage — a definition no longer deeply rooted in popular understandings of marriage — started to look like excluding a minority group out of ill will.

And it must be admitted that there was plenty of ill will. Eve Tushnet, herself an opponent of same-sex marriage, noted some years ago that

the ways in which homosexuality is stigmatized in our culture look nothing like the ways we treat many other things Scripture calls vices. Kids on the playground taunt each other for being gay, even disparaging other kids’ backpacks or pencil cases as “so gay.” People get beaten up or harassed on the street for their real or perceived homosexuality. Parents reject their children for coming out. . . . This isn’t how we treat the acts we really consider sinful. It’s how we treat scapegoats.

To the considerable extent that these attitudes and practices are declining, we are well rid of them.

None of this means that social conservatives were wrong to resist same-sex marriage. They are sometimes chided because they did less to resist, say, no-fault divorce; but that issue might have played out very differently if it had come to America through court decrees, and if opponents had been given reason to worry that the law and culture would start treating them as bigots.

Justice Kennedy’s opinion in the same-sex-marriage case was more responsive to trends in the culture than to any legal argument. It does a better job of explaining why he wants the law to recognize same-sex marriage than of explaining why the Constitution commands this policy. The liberal justices, more interested in the result than in the reasoning, joined the opinion without adding any argument of their own.

He dwells on one of his favorite themes, “dignity,” which has under his influence become as handy a judicial tool for policymaking as the “right to privacy” once was. By keeping the government from treating people in certain ways — not letting it quarter troops in their homes, for instance — the Constitution genuinely does protect both dignity and privacy. It does not follow that judges have a free-floating authority to nullify laws that they consider in conflict with those goods.

Kennedy’s emphasis on dignity suggests that what he is concerned with is the symbolic content of marriage policy: what it expresses. For him, what it should express, above all, is the value of romantic commitment. His decision was celebrated on social media with the apt slogan “Love wins.” Marriage, Kennedy authoritatively muses, has been “strengthened” by our changed understanding of it. You wouldn’t know it from marriage rates. The romantic conception of marriage has something going for it, which is why it has been triumphing for centuries, but it does seem to have made the institution more fragile.

Many Americans, tired of contention over same-sex marriage, have hoped that the Court would bring the argument to a close. That seems unlikely. We will now argue over how free opponents will remain to act on their beliefs. May the Knights of Columbus refuse to rent a wedding hall to a same-sex couple? Many liberals, gay and straight, will say no.

The argument that even bigots deserve tolerance has some force, but probably not enough, in our culture, to prevail. To preserve their freedom, traditionalists will need safety in numbers. The more religious groups soften their opposition to same-sex marriage, the more marginal the remaining opposition will be. The opponents will also have to guard the perception that they have a reasonable point of view.

Social conservatives will therefore have to keep making the argument against same-sex marriage — indeed, they will have to make it more forcefully than they have so far done.