On March 26 and 27, the U.S. Supreme Court will hear oral arguments in two cases challenging the definition of marriage as the union of one man and one woman. In Hollingsworth v. Perry, they will consider the constitutionality of the definition as enshrined in the California state constitution by voters in that state when they adopted “Proposition 8” in 2008 (effectively reversing the decision of the California Supreme Court to impose same-sex “marriage” earlier that year). In Windsor v. United States, they will consider the constitutionality of the same definition of marriage being adopted for all purposes under federal law through the 1996 Defense of Marriage Act (DOMA).

In anticipation of those oral arguments, I am offering a series of blog posts with questions and answers related to the issue. Today, we look at the question of whether the fact that not all opposite-sex couples reproduce undermines the argument that the public purpose of marriage is related to procreation.

Are you saying that married couples who don’t have children (whether by choice, or because of infertility or age) aren’t really married? If we deny marriage to same-sex couples because they can’t reproduce, why not deny it to those couples, too?

A couple that doesn’t want children when they marry might change their minds. Birth control might fail for a couple that uses it. A couple that appears to be infertile may get a surprise and conceive a child. The marital commitment may deter an older man from conceiving children with a younger woman outside of marriage. Even a very elderly couple is of the structural type (i.e., a man and a woman) that could theoretically produce children (or could have in the past). And the sexual union of all such couples is of the same type as that which reproduces the human race, even if it does not have that effect in particular cases.

Admittedly, society’s tangible interest in marriages that do not produce children is less than its interest in marriages that result in the reproduction of the species. However, we still recognize childless marriages because it would be an invasion of a heterosexual couple’s privacy to require that they prove their intent or ability to bear children.

There is no reason, though, to extend “marriage” to same-sex couples, which are of a structural type (two men or two women) that is incapable—ever, under any circumstances, regardless of age, health, or intent—of producing babies naturally. In fact, they are incapable of even engaging in the type of sexual act that results in natural reproduction. And it takes no invasion of privacy or drawing of arbitrary upper age boundaries to determine that.

Another way to view the relationship of marriage to reproduction is to turn the question around. Instead of asking whether actual reproduction is essential to marriage, ask this: If marriage never had anything to do with reproduction, would there be any reason for the government to be involved in regulating or rewarding it? Would we even tolerate the government intervening in such an intimate relationship, any more than if government defined the terms of who may be your “best friend?” The answer is undoubtedly “no”—which reinforces the conclusion that reproduction is a central (even if not obligatory) part of the social significance of marriage.

Indeed, the facts that a child cannot reproduce, that close relatives cannot reproduce without risk, and that it only takes one man and one woman to reproduce, are among the reasons why people are barred from marrying a child, a close blood relative, or a person who is already married. Concerns about reproduction are central to those restrictions on one’s choice of marriage partner—just as they are central to the restriction against “marrying” a person of the same sex.

Although not every opposite-sex couple reproduces, most do—and those that don’t are still able to provide both a mother and a father to any children they may adopt. Same-sex couples, on the other hand, can never reproduce as a natural result of their sexual intercourse, and deliberately deny either a mother or a father to any child they may raise. These undeniable and immutable differences provide clear, bright lines which easily justify classifying such couples differently under the law.