Just a generation ago, the idea of same-sex couples entering state-recognized marriages seemed inconceivable. Even as recently as 2003, private, consensual sodomy was still theoretically subject to criminal sanction in some American jurisdictions. Yet today, same-sex marriage is the law of the land in Massachusetts, as well as in Canada, South Africa, and a growing list of European countries. In the history of Western civilization, no formerly marginalized group has come so far, so fast.

Outside the most liberal parts of blue-state America, however, a strong majority of voters in the U.S. still oppose homosexual marriage. And in Washington, even Democrats regard the issue as a third rail. Yet many social conservatives worry that the intellectual ground is shifting. Defenders of traditional marriage tend to argue from their gut or their faith; neither carries weight in the courts, which is where such issues inevitably land. Proponents of same-sex marriage, on the other hand, make their case by appealing to the reigning principles of the age: tolerance, equality, and non-discrimination. A growing number of young Americans see this movement as their version of the civil-rights struggle.

If America does resist the movement, it will be thanks to thinkers like David Blankenhorn, a self-described liberal and life-long Democrat. In his new book, The Future of Marriage, he makes a principled case for the traditional definition of marriage. Crucially, he manages to do so while avoiding any hint of what liberal critics would call homophobia.

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As the founder and president of the New York-based Institute for American Values, Blankenhorn has made it his life’s work to strengthen marriage. As he sees it, the fallacy at the heart of the ongoing debate over homosexual marriage is that the issue turns on the question of love. He believes instead that any discussion of same-sex unions must begin with an understanding of the institution’s historical role, not the ardor of those who seek to enlist in it.

On this score, a survey of the American ideological landscape leaves Blankenhorn dismayed. Very few of those who purport to make serious arguments about same-sex marriage bother to research the history of what John Locke called the “first Society.” Instead, they casually define marriage with such flowery bromides as “a sacred bond,” “a unique and profound friendship,” or “a personal journey.” Dumbing marriage down to the level of twinkle-in-the-eye romantic fondness, Blankenhorn argues, flies in the face of its abiding purposes.

What we now know as marriage evolved gradually over the last five millennia. Prior to its invention, the concept of fatherhood was unknown in most ancient societies, except in the strict biological sense. Men would stumble into sexual encounters through rape, bribery, or seduction, and then go their way. The typical mass-burial site from this period, Blankenhorn notes, includes genetically related women and their offspring but, tellingly, no paterfamilias.

It was in the early cities of Mesopotamia and the Nile Valley that men began to take on a more active domestic role. Artifacts from the period depict men and women locked in tender embraces, as opposed to earlier, more pornographic images portraying ritual prostitution. Rulers of the era promulgated detailed laws to clarify the obligations of spouses. Four thousand years ago, for instance, the Sumerian ruler Lipit-Ishtar decreed that if a “harlot” bore children to a man whose own wife was barren, they would be his heirs.

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Marriage has taken many forms in human societies, but, as Blankenhorn explains, all of them have shared several essential elements, including an expectation of sexual intimacy and shared responsibility for the care of offspring. Moreover, just about every known society conforms to what anthropologist Bronislaw Malinowski called the “principle of legitimacy,” whereby children and the mothers who bear them suffer a diminution in status if they are not joined by a father. In other words, marriage historically has been not a “personal journey” but the essential institution marking the establishment of a new household and family. Even the most primitive societies, Blankenhorn writes, have understood what we have forgotten: that every child deserves a mother and a father.

Setting out this anthropological background takes up the first half of the book. By this point, it is obvious that Blankenhorn’s overarching concern is not same-sex marriage per se but rather the broader, decades-long phenomenon whereby—thanks to no-fault divorce, secularism, falling birth rates, and the cult of individualism—the “institutionality” of marriage has eroded in America.

As Blankenhorn sees it, homosexual marriage is not a cause but another symptom of this problem: when marriage is seen as nothing more than a purely personal attestation of love between two consenting adults, disconnected from its historic role as the bedrock of families, why should homosexuals not see their exclusion as arbitrary? “The great challenge of our time,” he thus concludes, “lies neither in defending nor in thwarting same-sex marriage” but rather in “renew[ing] marriage as a powerful way of living that calls forth and reflects the best in us, that successfully meets important social needs, and that is worthy of strong social support.”

Blankenhorn believes that homosexual couples have a compelling claim to equal treatment, and, as a liberal, he is plainly agonized by the idea that they view exclusion from marriage as a challenge to their self-worth and dignity. But ultimately, he argues, the purpose of marriage is not to provide a social seal of approval to homosexual love, to encourage diversity, or to facilitate exercises in assisted reproduction. “If we could [strengthen marriage] by embracing same-sex marriage,” he writes, “I would gladly embrace it.” But if we remove the requirement that marriage is between a man and a woman, who, he asks, is to say it has to be between two people? Or to be sexually exclusive? Or sexual at all? Marriage could end up as just another “lifestyle” choice, stripped of the special status our forebears have conferred on it since the rise of settled human civilization.

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The Future of Marriage is a book that both sides of the debate over same-sex marriage should take seriously. Culture-war hardliners who believe homosexuality is inherently immoral, or who view marriage as a hopelessly outdated and heterosexist institution, will dismiss Blankenhorn’s hand-wringing as beside the point. But for those wrestling in good faith with the issue of how to reconcile the institution of marriage with the rights of homosexuals, Blankenhorn’s analysis should be welcome.

The question is whether his argument can win the day. Blankenhorn’s main problem is that the underlying cultural phenomenon that he criticizes—the privileging of individual identity and self-expression over society’s strictures—is so dominant and deep-seated that it is hard to imagine its reversal. It certainly will not yield to the laundry list of policy recommendations he offers in his closing pages: tweaking the tax code to help married couples, premarital education, church-based marriage mentoring, etc. As things stand, the only thing blocking the widespread adoption of same-sex marriage in America is the country’s Christian religiosity, with its attendant suspicion of homosexuality itself.

Blankenhorn would argue that even a secularist should oppose a social innovation that will compromise the well-being of children by denying them a conjugally joined mother and father. But notwithstanding the wealth of historical examples he cites, it is not clear how his empirical claims about family structure bear on the modern movement for same-sex marriage. Although decades’ worth of studies show that children raised in single-parent homes suffer an elevated risk of everything from school delinquency to suicide, data are sparse on children raised in two-parent homosexual households. And what we do know from Canada and Europe does not indicate that the sky is falling. Anecdotal evidence suggests that those self-selected homosexual couples who tie the knot skew upward in education and wealth—measures that correlate strongly with positive childhood outcomes—and embrace the traditional norms associated with bourgeois married life.

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But If Blankenhorn has not made the definitive case for opposing same-sex marriage, he has at least set out an intelligent framework for debating the matter. Proponents would like to cast the issue as one of tolerance versus bigotry. But as The Future of Marriage demonstrates, it is more properly thought of as a complicated balancing act involving not only the interests of homosexuals and children but also society’s need to maintain the strength of an essential 5,000-year-old institution.

This distinction is crucial when one considers that it was judges who summoned same-sex marriage into being in both Massachusetts and Canada. Notwithstanding the claims of litigious activists, such divisive issues are best decided in legislatures, not in courtrooms. As the example of Roe v. Wade demonstrates, judges who arrogate to themselves the role of social engineers do so at the cost of the judiciary’s prestige and the legitimacy of the policies they dictate.

In time, as evidence on the issue accumulates, elected lawmakers across the United States may or may not decide that the leap into same-sex marriage is a risk worth taking. In either case, that determination should be left to them and their constituents.