The U.S. Supreme Court decides next week whether to hear challenges to laws defining marriage as the conjugal union of a man and a woman. It does so after two different electoral outcomes. In May, North Carolinians voted to amend their state constitution to protect the conjugal definition of marriage, a definition that 41 states retain. But on Nov. 6, voters in Maine, Maryland and Washington state endorsed a revisionist view of marriage as the union of any two adults.

How should the Supreme Court decide? How should voters?

We can't move one inch toward an answer simply by appealing to equality. Every marriage policy draws lines, leaving out some types of relationships. Equality forbids arbitrary line-drawing. But we cannot know which lines are arbitrary without answering two questions: What is marriage, and why does it matter for policy?

The conjugal and revisionist views are two rival answers; neither is morally neutral. Each is supported by some religious and secular worldviews but rejected by others. Nothing in the Constitution bans or favors either. The Supreme Court therefore has no basis to impose either view of marriage. So voters must decide: Which view is right?

As we argue in our book "What is Marriage? Man and Woman: A Defense," marriage is a uniquely comprehensive union. It involves a union of hearts and minds; but also—and distinctively—a bodily union made possible by sexual-reproductive complementarity. Hence marriage is inherently extended and enriched by procreation and family life and objectively calls for similarly all-encompassing commitment, permanent and exclusive.

In short, marriage unites a man and woman holistically—emotionally and bodily, in acts of conjugal love and in the children such love brings forth—for the whole of life.

These insights require no particular theology. Ancient thinkers untouched by Judaism or Christianity—including Aristotle, Plato, Socrates, Musonius Rufus, Xenophanes and Plutarch—also distinguished conjugal unions from all others. Nor did animus against any group produce this conclusion, which arose everywhere quite apart from debates about same-sex unions. The conjugal view best fits our social practices and judgments about what marriage is.

After all, if two men can marry, or two women, then what sets marriage apart from other bonds must be emotional intensity or priority. But nothing about emotional union requires it to be permanent. Or limited to two. Or sexual, much less sexually exclusive. Or inherently oriented to family life and shaped by its demands. Yet as most people see, bonds that lack these features aren't marriages.

Far from being "slippery slope" predictions, these points show that the revisionist view gets marriage wrong: It conflates marriage and companionship, an obviously broader category. That conflation has consequences. Marriage law shapes behavior by promoting a vision of what marriage is and requires. Redefinition will deepen the social distortion of marriage—and consequent harms—begun by policies such as "no-fault" divorce. As marital norms make less sense, adherence to them erodes.

Conservative scaremongering? No. Same-sex marriage activist Victoria Brownworth, like other candid revisionists, says that redefinition "almost certainly will weaken the institution of marriage," and she welcomes that result.

Yet weakening marital norms will hurt children and spouses, especially the poorest. Rewriting the parenting ideal will also undermine in our mores and practice the special value of biological mothers and fathers. By marking support for the conjugal view as bigotry, it will curb freedoms of religion and conscience. Redefinition will do all this in the name of a basic error about what marriage is.

Some bonds remain unrecognized, and some people unmarried, under any marriage policy. If simply sharing a home creates certain needs, we can and should meet them outside civil marriage.

Moreover, if we reject the revisionist's bare equation of marriage with companionship—and the equation of marriage licenses with all-purpose personal approval—we'll see that conjugal marriage laws deprive no one of companionship or its joys, and mark no one as less worthy of fulfillment. (Indeed, using marriage law to express social inclusion might further marginalize whoever remains single.)

True compassion means extending authentic community to everyone, especially the marginalized, while using marriage law for the social goal that it serves best: to ensure that children know the committed love of the mother and father whose union brought them into being. Indeed, only that goal justifies regulating such intimate bonds in the first place.

Just as compassion for those attracted to the same sex doesn't require redefining marriage, neither does preserving the conjugal view mean blaming them for its erosion. What separated the various goods that conjugal marriage joins—sex, commitment, family life—was a sexual revolution among opposite-sex partners, with harmful rises in extramarital sex and nonmarital childbearing, pornography and easy divorce.

Only when sex and marriage were seen mainly as means to emotional satisfaction and expression did a more thorough and explicit redefinition of marriage become thinkable—for the first time in human history. The current debate just confronts us with the choice to entrench these trends—or to begin reversing them.

That debate certainly isn't about legalizing (or criminalizing) anything. In all 50 states, two men or women may have a wedding and share a life. Their employers and religious communities may recognize their unions. At issue here is whether government will effectively coerce other actors in the public square to do the same.

Also at issue is government expansion. Marital norms serve children, spouses, and hence our whole economy, especially the poor. Family breakdown thrusts the state into roles for which it is ill-suited: provider and discipliner to the orphaned and neglected, and arbiter of custody and paternity disputes.

For all these reasons, conservatives would be ill-advised to abandon support for conjugal marriage even if it hadn't won more support than Mitt Romney in every state where marriage was on the ballot.

They certainly shouldn't be duped into surrender by the circular argument that they've already lost. The ash-heap of history is filled with "inevitabilities." Conservatives—triumphant against once-unstoppable social tides like Marxism—should know this best. "History" has no mind. The future isn't fixed. It's chosen. The Supreme Court should let the people choose; and we should choose marriage, conjugal marriage.

Mr. Girgis is a Yale law student and doctoral student in philosophy at Princeton. Mr. Anderson is a fellow at the Heritage Foundation. Mr. George is professor of jurisprudence at Princeton and a visiting professor at Harvard Law School. Their book, "What Is Marriage? Man and Woman: A Defense," will be published in December by Encounter Books.

A version of this article appeared November 21, 2012, on page A17 in the U.S. edition of The Wall Street Journal, with the headline: The Wisdom of Upholding Tradition.