Yesterday, I placed the debate about same-sex marriage within a larger historical context focusing on concern for marriage. Today, I want to home in on the question of what marriage is. This is the question that most proponents of redefining marriage to include same-sex relationships keenly want to avoid, to shuffle offstage as soon as it’s brought up.

Those who would redefine marriage employ two evasion techniques. First, they might appeal to historical inevitability as a reason not to have to answer the question of what marriage is—as if it were a moot question, already decided. My Heritage Foundation colleague Andrew T. Walker and I respond to some of those claims at National Review this morning.

The second evasion technique is to appeal simply to equality—“marriage equality,” after all, has been good sloganeering. But it’s sloppy reasoning.

Why? Well, every law makes distinctions. Equality before the law protects citizens from arbitrary distinctions, from laws that treat them differently for no good reason. But in order to know if a law makes the right distinctions—if the lines it draws are justified—you have to know the public purpose of the law, and the nature of the good being advanced or protected.

Just ask yourself: If the law recognized same-sex couples as spouses, would it still fail to respect the equality of citizens in multiple-partner relationships? Are those inclined to such relationships being treated unjustly when their consensual romantic bonds go unrecognized, their children thereby “stigmatized,” their tax filings unprivileged?

This isn’t scaremongering. In 2009, Newsweek reported that there were over 500,000 polyamorous households in America. And prominent scholars and LGBT activists have called for “marriage equality” for multipartner relationships since at least 2006.

And in any case, the question is more fundamental: Once one jettisons sexual complementarity—the bodies of men and women go together—what principle can one offer to limit civil marriage to monogamous couples? For that is the only way to answer the charge that withholding a “fundamental right” from even just one multiple-partner household isn’t a grave injustice.

Again, to know when the lines drawn by a marriage law are arbitrary—when they violate equality—we have to know what marriage is and why the state promotes it. Tomorrow’s post will examine that latter question; today we focus on what marriage is.

Consider a favorite analogy of supporters of redefinition: Laws defining marriage as a union of a man and woman are unjust—fail to treat people equally—exactly like laws that prevented interracial marriage.

Such appeals simply beg the question of what is essential to marriage. They just assume exactly what’s in dispute here: that gender is just as irrelevant as race. It is true, of course, that the color of two people’s skin has nothing to do with what kind of bond they have. But the sexual difference between a man and a woman is central to what marriage is. Men and women—regardless of their race—can unite in marriage; and children need moms and dads—regardless of their race. You can’t know either fact, though, without at least a rough idea of what, essentially, makes a marriage.

My co-authors and I present arguments for marriage as the union of husband and wife—and against objections to that view—in our new book, What Is Marriage? Man and Woman: A Defense. As we argue there, 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 complementarity. As the act by which spouses make marital love also makes new life, so marriage itself is inherently extended and enriched by family life and 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.

This understanding—and only this one—explains the key features of marriage. If marriage isn’t founded on a comprehensive union made possible by the sexual complementarity of a man and a woman, then why can’t it occur among more than two people? If marital union isn’t founded on such sexual acts, then why ought it be sexually exclusive? If marriage isn’t a comprehensive union and has no intrinsic connection to children, then why ought it be permanent?

So to those who take the opposing viewpoint, the challenge is to find a coherent set of answers to these questions:

 —  Ryan T. Anderson is the William E. Simon Fellow at the Heritage Foundation’s DeVos Center for Religion and Civil Society.