With the war in Afghanistan not yet over and the economy still reeling from the Great Recession, who would have predicted that 2012 would be the year of social issues? But so it is proving to be, between Rick Santorum’s surprisingly strong performance in the Republican primaries, the Obama administration’s mandate for employer-provided health insurance to cover contraception, and—in a series of battles in legislatures from New Jersey to Maryland—the ongoing struggle over same-sex marriage. Where the last is concerned, polls indicate that while more Americans still oppose gay marriage, the majority that does so is dwindling rapidly.

Rod Dreher has made the case that advocates of same-sex marriage are the aggressors on this front of the culture war. But they have presented their argument as a defensive one—a matter of equal rights being unfairly violated—and this has been a masterstroke of propaganda, winning much sympathy for their cause. The thing against which supporters of same-sex marriage have committed their aggression is tradition, specifically the West’s traditional understanding of marriage dating back a thousand years and more.

“Same-sex marriage has only been on the national radar since 1993, when a Hawaii court ruled that the state had to demonstrate just cause for why marriage ought to be denied to same-sex couples,” Dreher wrote on his blog at The American Conservative’s website. “That was fewer than 20 years ago, and in that time, support for same-sex marriage has increased at a pace that is nothing short of revolutionary. According to the trajectory of polling, at some point in the next few years, what had been the settled view of the nature of marriage for millennia will have been rejected by a majority of the American people.”

How has this happened? The gradual triumph of gay marriage is not merely due to a legal change that began 20 years ago or even to the sexual revolution of a half-century past; rather it is a consequence of a shift in the foundations of Western civilization that has been taking place over centuries—a shift from Christian to liberal foundations. So profound is this transformation that even the opponents of same-sex marriage are not exactly fighting to recover the old way of life.

To understand how marriage has changed, and not changed, over the course of Western history one can hardly do better than turn to Harvard sociologist Carle Zimmerman’s Family and Civilization as a primer. First published in 1947, it remains an invaluable, indeed prophetic, guide to the marriage debate and wider culture wars. While same-sex marriage may be an absolute novelty, there have been pitched battles over the definition of marriage before, as when the Catholic Church told the barbarians who had overtaken the Roman Empire that they could not continue their practices of cousin marriage—a tradition from time immemorial—if they wished to be Christians.

Indeed, as Zimmerman writes, “in the course of seven or eight centuries the family system of Europe had twice completely reversed its trend” thanks to the Church, which first reformed the socially atomistic conjugal practices of the late Romans before tackling the blood-bound “trustee” families of the invading tribes. “This struggle, one of the most interesting in the history of the Western family, is relatively unknown to us today,” though it was a matter of civilization-shaping importance at the beginning of European Christendom.

The balance between the social extremes of atomism and tribalism could only be maintained as long as the Church was the primary authority responsible for marriage—which it was for over a thousand years. “The barbarian family had to be broken away from clan influences and brought under that of the church,” writes Zimmerman, but “if temporal forces and strong states could take from the church its power, rule, and regulation of the family, then the atomistic type could reappear. Actually, this is what happened.”

Even Zimmerman could not have anticipated same-sex marriage, but he might not have been surprised by it. As Christianity has lost its power in public life, so too have the forms of marriage and family that it established given way to new configurations shaped by the institutions and ideologies that hold power today—specifically, liberalism and the modern state. But did liberalism, with its bedrock principle of legal equality for all individuals, have to lead to gay marriage?

Same-sex marriage is a radically new notion; its apologists have to stretch exiguous evidence to find any foreshadowing in past societies. This should not be surprising, since homosexuality itself, as a thing parallel to heterosexuality, is a recent invention. Homosexual activity may be as old as civilization, but the idea of a category of person whose sexual identity is primarily defined by same-sex attraction, yet who is otherwise quite like the mainstream of society, is of recent vintage. That people in this group are not negligible in numbers—amounting to perhaps 5 percent of the general population—has also been a slowly dawning realization.

Once society was widely conscious of this population, and had an inkling of its extent, there was no question of reverting to the status quo ante. The knowledge itself had changed the political question. Not only were homosexuals not going back into the closet, but the rest of society could not forget that they exist. And there had been little in the way of a “traditional” approach to something that was beyond the margin of public consciousness. So now the question arose of how to think about—and act toward—this alarming new population. Should it be included in or excluded from the body politic, and on what terms?

At first, exclusion won out: more laws against sodomy were added to the books in the early part of the 20th century. In many jurisdictions, statutes had to be revised to criminalize what before had not even been known. Politicians passed laws; doctors pronounced the condition a disease. There was from the beginning little thought to equal protection under the law.

But in the latter half of the 20th century two things steadily eroded the cultural and legal taboos against homosexuality. The first was that it had come to be seen as an innate desire about which individuals have little choice. The second was that as these strange new beings emerged from their hiding places they didn’t look so frightening—indeed, they looked a lot like everybody else. The great public-relations victory won by the gay-rights movement that hastened the advent of gay marriage was the shift in the 1990s away from a radical, anti-bourgeois image toward one more in keeping with societal norms, from the militancy of ACT-UP to the banality of “Will and Grace.”

The gay-marriage effort has been a cause as well as an effect in this change: while same-sex marriage is disturbing to many Americans, it is reassuring to others, suggesting as it does loyalty to a middle-class ideal. Those homosexuals who remember more radical days are often dismissive of bourgeois aspirations of the younger set. As the gay libertarian Justin Raimondo has written in these pages: “The modern gay-rights movement is all about securing the symbols of societal acceptance. … But if ‘gay pride’ means anything, it means not wanting, needing, or seeking any sort of acceptance but self-acceptance. Marriage is a social institution designed by heterosexuals for heterosexuals: why should gay people settle for their cast-off hand-me-downs?”

But this radical approach, aiming at neither official approval nor persecution but the preservation of “otherness,” lost out to the power of cultural assimilation. Americans are highly conformist, even the homosexuals among them. Meanwhile the repressive approach favored by almost all Americans before the 1960s—it was hardly a distinctively conservative position—has collapsed in the face of reality. Medicalizing or criminalizing homosexuals away was never going to work: what would be achieved after all that effort—the therapy programs, the prison terms, the tax dollars to pay for everything?

Yet if homosexuals were not going to be under legal or therapeutic penalty, what would become of them? This is the question to which few conservatives can supply a satisfactory answer because the principles that conservatives affirm point toward policies that conflict with their wishes.

If a conservative continues to endorse the pre-1960s mentality—itself a modern mentality, quite different from that of, say, the 1760s—then opposing gay marriage and banning homosexuals from serving in the military is not nearly enough. How can such people be too corrupting to be trusted in the barracks or on the battlefield yet be deemed safe for schools? Sen. Jim DeMint (R-S.C.) was guileless enough to express this logic in a 2004 election debate: “We need the folks that are teaching in schools to represent our values,” he said, agreeing that this definition did not include homosexuals. (He later added, in a spirit of fairness, that it also did not include “a single woman, who was pregnant and living with her boyfriend.”)

Consistently applied, this perspective leads to the conclusion that anti-sodomy laws are of some importance, thus Lawrence v. Texas was not merely a moot exercise in judicial activism but a substantive blow to virtue and justice, and at a minimum homosexuals should be discriminated against in public and private employment. Yet this is more than many social conservatives are willing to contemplate, and it most certainly is not something that Republican politicians more discreet than DeMint are willing to voice.

The second consistent position that conservatives can embrace, however reluctantly, would be that of providing full legal equality. This could be seen as a capitulation to liberalism; it could also be seen as an acknowledgement of reality. The trouble with this position is that it doesn’t stop where most conservatives would like it to stop: the logic of legal equality certainly demands that homosexuals be allowed to serve in government, including in the military, and prima facie it demands that they be afforded equal access to the institution of marriage. Conservatives can try to draw the line before that point, but doing so requires making an exception to the principle of legal equality, and exceptions are, by their very nature, more difficult to establish than arguments that go along with general rules.

There is an argument that can be made against same-sex marriage on grounds that have nothing to do unequal treatment of homosexuals. Namely, the benefits to society that arise from the institution of marriage—the reasons we keep the institution around after a few thousand years—do not apply with much force to same-sex unions. Historically, marriage was the institution that conferred legitimacy, giving wives legal privileges above mistresses and concubines and according legitimate children rights to property and higher status than illegitimate ones. Vestiges of this ancient household basis for marriage still remain. But they aren’t much of an issue with same-sex marriage since children only arise in such contexts from very deliberate plans—which cannot be said about the appearance of children in heterosexual relationships.

The broader social interest in limiting promiscuity and encouraging bourgeois life doesn’t apply to gay marriage for reasons having to do with the differences between the sexes themselves, not differences in sexual orientation. Men, heterosexual or homosexual, are generally promiscuous. Women, heterosexual or homosexual, tend to be less so. Heterosexual marriage reduces promiscuity precisely because it applies the less promiscuous nature of the wife to limiting the promiscuity of the husband. Two men cannot on average be relied upon to check one another’s promiscuity. Many notable homosexuals are quite honest about this; the writer and activist Dan Savage, for example, who wishes to make monogamy less essential to marriage for heterosexuals and homosexuals alike, last year told the New York Times, “The mistake that straight people made was imposing the monogamous expectation on men. Men were never expected to be monogamous.”

Two women, by contrast, do not need the legal force of marriage to maintain a monogamous relationship: lesbians are already more monogamous than heterosexual couples. In practical terms, so far as checking promiscuity is concerned, marriage is superfluous for lesbians and not very effective for homosexual men. To the extent that marriage serves as a brake on promiscuity at all, this is owing to the sex differences of the spouses.

The weakness of this argument is that it’s still exceptional: does it present a strong enough case for making an exception to legal equality? Probably not for most Americans, who cherish the sentimental side of marriage—the individualistic and emotive side—over its institutional and biological basis.

Social conservatives are caught between two worldviews, each of which they are reluctant to endorse fully. And with good reason: neither is a genuinely traditional because the traditional world—the Christian civilization to which social conservatives look as their ideal—has already given way to something radically new, leaving traditionalists with a choice between modern alternatives of left and right, neither of which is wholly in accord with the old values.

Are the stakes as high as they think? Same-sex marriage will not lead to civilizational collapse; the social atomism of which it is a symptom is more likely to do that. But there are tough questions about how nondiscrimination and public-accommodations laws will be applied against religiously affiliated institutions, even if churches themselves are exempt from having to participate in the public status of same-sex marriage. Traditionalists are right to be worried: religious liberty too is treated as an exception to liberalism, one for which powerful arguments must be made and which always faces an uphill battle. But the key problem here may not be whether or not there’s gay marriage, but the reach of non-discrimination and public-accommodations law.

Social conservatives have a hard time tackling those concerns, however, because of the inherited guilt they feel over the retrograde views that many past conservatives held about legal equality for racial minorities. Social conservatives are also fearful of being demonized as racist in the way that the libertarian Congressman Ron Paul and his son Sen. Rand Paul have been when they have made arguments against nondiscrimination laws. (Barry Goldwater once made much the same arguments.) This is a difficult knot, since conservatives who are not comfortable taking the Goldwater-Paul position against all nondiscrimination law must again make an exception to argue that it shouldn’t apply where homosexuals and their marriages are concerned.

But that’s the pot in which social conservatives are being boiled. They have made enough concessions to the reality of political life in 21st-century America—to the principle of legal equality and the need for some nondiscrimination law—that they’re left making largely unsympathetic and unconvincing arguments for exceptions. Over time they may feel compelled throw their full electoral weight behind the libertarian principle of tolerance even for intolerance as the only viable alternative to a futile authoritarianism or outright surrender to liberalism. From libertarians they might also take the lesson that just because something is enshrined in law does not mean it has thereby acquired a higher moral status.

For the better part of 2,000 years the meaning of marriage was stable because the authority of Christianity was constant. In an age when the Church no longer supplies the institutional framework for family life, the definition of marriage has become radically uncertain. Into this fluid environment—in which individualism or atomism is once again rising to the fore, as in the Roman Empire—the new fact of homosexuality was gradually introduced. The result, same-sex marriage, has shocked conservatives. But this innovation has moved so far so quickly only because it is not at all out of step with the institutions and ideas of our time.

Daniel McCarthy is editor of The American Conservative.