Why is the government in the marriage business? As I explain in Truth Overruled: The Future of Marriage and Religious Freedom, it’s to encourage a man and a woman to unite as husband and wife to then be mother and father to any children their union produces. It’s based on the anthropological truth that men and women are distinct and complementary, the biological fact that reproduction requires a man and a woman, and the social reality that children deserve a mother and a father.

Perhaps the most common objection to this basic argument involves infertility. If infertile couples can marry—and no one has ever denied that they can—how can the definition of marriage be linked to procreation? Proponents of same-sex marriage usually regard this argument as a “silver bullet” that destroys the traditional understanding of marriage—as if no one in the previous millennia has realized that some couples (and any woman above a certain age) can’t conceive a child.

But as I explain in my new book, there are four responses to this argument.

First, as a policy matter, the state is in the business of recognizing marriage not because every marriage will produce a child but because every child has a mother and a father. Through its marriage policy, the state respects the natural bonds that unite the parents who brought a child into the world and encourages them to commit to each other permanently and exclusively. Public policy must consider the big picture, not individual cases. It is the procreative nature of marriage rather than the actual procreative results of individual marriages that explains government policy in this area. (And would anyone really want the government to require fertility tests or to ask couples if they intend to have children?)

Second, as a practical matter, many couples who think they are infertile end up conceiving or adopting children. Many who say they never want children change their minds. It’s important to keep these men and women united with each other. Indeed, infertility rarely strikes both husband and wife, and marital fidelity ensures that the fertile spouse doesn’t procreate children with someone else, children who will be deprived of a fully committed mother and father. The 50-year-old husband whose wife has gone through menopause will never beget children with another woman if he’s faithful to his marriage vows. The state has a general interest in channeling their sexual desire into marriage.

Third, as a philosophical matter, an infertile marriage is fully a marriage. As I explain in Truth Overruled, a marriage is a comprehensive union marked by one-flesh union—the coordination of the spouses’ two bodies toward the single biological end of reproduction. That coordination—and thus the one-flesh union—takes place whether or not it achieves its biological end in the fertilization of an egg by a sperm some hours later. The union, like the act that seals it, is still oriented toward family life. This explains why in common, civil, and canon law, infertility has never nullified a marriage. Impotence, by contrast—which prevents a couple from consummating their union in the one-flesh marital act—has been grounds for declaring that a marriage was never completed.

Fourth, as a pedagogical matter, recognizing marriages in spite of infertility teaches that marriage is a comprehensive union, not merely an instrument for baby making. That teaching benefits society by encouraging genuine devotion—and hence stability—in all marriages. By contrast, redefining marriage to include same-sex relationships will teach that marriage (gay or straight) is an instrument for gratifying the emotions of adults. The stability that guarantees children a mom and a dad is not a component of such a union.

In sum, then, public policy is about the rule not the exception, marital norms benefit society even when lived out by infertile couples, infertile marriages are still marriages, and state recognition of infertile marriages has the benefit of reinforcing the truth about marriage without any disadvantages.

This essay is adapted from the book Truth Overruled: The Future of Marriage and Religious Freedom.