A few years ago, MTV ran a short cartoon segment that began with two men and two women dressed in traditional wedding attire. As a bridal chorus played on a pipe organ in the background, the two brides and the two grooms reached out to hold hands with each other. “Watch and Learn,” the 15-second spot advised. The commercial was, in many ways, just a sign of the times. Massachusetts had begun granting marriage licenses to same-sex couples the previous year, and seven other states, along with the District of Columbia, have since followed suit. The result is a patchwork of state marriage laws, recognizing (or not), in varying degrees, the same-sex marriages performed in other states.

This splintered arrangement may soon change. The Ninth Circuit Court of Appeals recently held that California’s traditional definition of marriage, approved by a voter referendum in 2008, is unconstitutional under the Fourteenth Amendment’s Equal Protection and Due Process clauses, and some legal commentators expect that the Supreme Court will soon take the opportunity to overturn the traditional marriage laws remaining on the books in forty-three states. Such a prospect, as a matter of constitutional law, would have been unthinkable only a decade or two ago. What happened?

The story—or at least part of it—begins with Estelle Griswold, Executive Director of the Planned Parenthood League of Connecticut in the 1960s. After being arrested for providing contraception to a married couple in violation of a rarely enforced state law, Griswold appealed her conviction to the Supreme Court. The result was Griswold v. Connecticut (1965), a landmark decision that located a constitutional right to privacy in the sacredness of marriage itself. As Justice William O. Douglas maintained in his opinion for the Court, “We deal here with a right to privacy older than the Bill of Rights—older than our political parties, older than our school system. Marriage is a coming together for better or for worse, hopefully enduring, and intimate to the degree of being sacred.” Connecticut’s century-old anti-contraception statute, Douglas asserted confidently, was an affront to the “sacred precincts of the marital bedroom.”

So framed, the constitutional right to privacy was bound up with a fairly traditional understanding of marriage. Former Kennedy Administration labor secretary Arthur Goldberg even took the occasion to emphasize, in a concurring opinion, that the Court’s holding “in no way interferes with the state’s proper regulation of sexual promiscuity or misconduct.” “‘Adultery, homosexuality and the like,’” Goldberg insisted, quoting a previous opinion by John Marshall Harlan, “are sexual intimacies which the State forbids . . . but the intimacy of husband and wife is necessarily an essential and accepted feature of the institution of marriage.” In a 1961 case involving the same Connecticut statute, Harlan had suggested that laws “confining sexuality to lawful marriage form a pattern so deeply pressed into the substance of our social life that any Constitutional doctrine in this area must build upon that basis.”

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Following a long tradition, the Court simply took for granted that “husband and wife” were necessary constituents of a marital union, and marriage itself provided the foundation for whatever constitutional right to sexual privacy existed. As one might suspect, this theoretical framework was short-lived. “It is true that in Griswold the right of privacy in question inhered in the marital relationship,” a majority of justices would declare just a few years later. “Yet the marital couple is not an independent entity with a mind and heart of its own, but an association of two individuals each with a separate intellectual and emotional makeup.” The upshot, moving into the 1970s, was that constitutional privacy—whatever it entailed—was the right of an individual, married or not.

Unmoored from its origins in marriage, the Court soon exalted the individual right to privacy—understood increasingly as a right to abortion and sexual autonomy—to a preeminent place in constitutional law. Over time, several of the justices began to emphasize “liberty” or “liberty interests” (rather than privacy), and, in the rhetorical culmination of the line of reasoning coming out of Griswold, the Court went on to claim that “the right to define one’s own concept of existence, of meaning, of the universe, and of the mystery of human life” is at the very heart of the personal liberty protected by the Fourteenth Amendment.

The most recent such pronouncement came in Lawrence v. Texas (2003), a decision that overturned the Lone Star State’s traditional proscription of intimate homosexual acts. Clarence Thomas undoubtedly spoke for many conservatives when he described the Texas statute as an “uncommonly silly” use of “valuable law enforcement resources,” but the real import of the decision came in its logic and reasoning. What began as a right to privacy, older than the Bill of Rights and deeply rooted in the sacred bonds of marriage, had ended, nearly four decades later, in an atomistic and libertine conception of liberty that eroded what Justice Goldberg, forty years before, had underscored as “the state’s proper regulation of sexual promiscuity or misconduct.”

The new conception of liberty underpinning Lawrence made biological sex irrelevant to coupling and dissociated sex from reproduction, child-rearing, and the like—leaving traditional proscriptions of non-marital sex standing on shaky constitutional ground. And so it would seem, as Antonin Scalia noted in dissent, that the Court’s opinion also dismantled “the structure of constitutional law that has permitted a distinction between heterosexual and homosexual unions, insofar as formal recognition in marriage is concerned.”

Although Lawrence clearly laid the precedent for a future Supreme Court decision mandating state recognition of same-sex marriage, the doctrinal development must also be understood within the context of the quickly changing cultural and political dynamics at the state level. Since the mid-1990s, political battles over marriage have been waged from state to state, and the pendulum has swept from state constitutional amendments enshrining traditional definitions of marriage (31 states) to judicial or legislative redefinitions of marriage to include same-sex couples (seven states and the District of Columbia). Along the way, the media have championed and public opinion has steadily warmed to the idea of same-sex marriage. Last May, Gallup reported for the first time that a majority of all Americans—and 70 percent of 18- to 34-year-olds—agree with the statement “same-sex marriages should be recognized by the law as valid, with the same rights as traditional marriages.”

The upward trend of support for same-sex marriage—in tandem with the doctrinal developments emanating from Griswold—has given rise to the now-familiar assertion that defenders of traditional marriage are living on the wrong side of history. Despite the Marxist tinge, this frequent claim about history has less to do with some Hegelian dialectic than with a sense that public opinion will soon settle the issue and leave partisans of the old order as social pariahs. “In America,” Tocqueville noted nearly two centuries ago, “the majority raises formidable barriers around the liberty of opinion; within these barriers an author may write what he pleases, but woe to him if he goes beyond them.” This, it seems, is the subtle warning behind the “wrong side of history” charge: soon the law and public opinion will be fixed, and those who once spoke out in defense of traditional marriage will be scorned in public life and unwelcome in polite company.

Whether the Supreme Court or history will settle on same-sex marriage in this way remains to be seen, but there is one reason to suspect it will not: the rationale for same-sex marriage undercuts the very reasons why we have a public institution called “marriage” in the first place. The old definition of marriage supposed that men and women were complementary by nature and that the state, barring some case of neglect or abuse, had an interest in legally binding fathers and mothers to each other and to their children. On the old account, to say marriage was between a man and a woman simply followed from what marriage, according to its nature and public purpose, is.

But any attempt to redefine the institution in a way that treats biological sex as fungible or irrelevant ends up making the sole requirement for participation something like erotic love between two consenting adults. If this is indeed the case, it prompts the questions—why erotic love? Why two people? Why does the state single this kind of relationship out for special protection? When we decide to take these questions seriously, the debate over same-sex marriage will either be a blip on the way to the abolition or privatization of marriage altogether or the occasion for us to reaffirm what Justice Goldberg had insisted in 1965: that marriage, involving the “intimacy of husband and wife,” is an “institution which the state not only must allow, but which always and in every age it has fostered and protected.”