The central argument in favor of legally recognizing same-sex relationships as marriages is a straightforward equality claim: because there is no relevant difference between the aptness (suitability, capacity) of same-sex couples and opposite-sex couples for marriage, restrictive marriage laws arbitrarily deny important recognition and benefits to same-sex couples who wish to marry. Restrictive laws are unconstitutional because they lack a rational basis for discriminating among ostensibly eligible couples.

These arguments depend, of course, on some account of what marriage is that welcomes same-sex couples without qualification. Proponents of same-sex marriage argue that same-sex marriage is mainly about adult satisfactions—love, affection, intimacy, mutual support—just as (they say) it is for most opposite-sex couples. In these proposals, children are strictly optional, a matter of private judgment. Children have nothing to do with the definition of marriage, or with those features of marriage that attract the law’s attention and earn marriage the law’s protection and support. Make no mistake about it: any such account as this is normative, a claim (or set of claims) about what marriage as such and properly understood is really and essentially about. Anyone who makes a pro- same-sex marriage argument dependent upon the state’s obligation to be morally neutral about the meaning of marriage misunderstands the nature of the argument.

A successful counterargument depends therefore on a valid understanding of marriage which implies that same-sex couples are not suited to marry. (For reasons I shall not discuss here, I think that any successful argument must have to do with the nature of marriage itself. Denying persons who may in moral truth marry the legal opportunity to do so for extrinsic reasons of policy or prudence is, I think, unjust.) Almost everyone on both sides recognizes that there is one sure winner here: if marriage can be shown—coherently, reasonably—to be a procreative relationship, then same-sex couples cannot marry. For whatever else they can do, no same-sex couple can produce fruit of their union. They cannot bring into being children of their own.

Courts, commentators, and activists have made several arguments against the procreative understanding of marriage. Some of these arguments are silly or disingenuous, or both. Others deserve (and have received) careful and cogent responses. In this essay I wish to take up one gap in this set of responses. Here I aim to answer two questions: Why is it morally significant that children come to be as fruit of their married parents’ sexual acts? And, why is it legally important? In other words, on what grounds does public authority base marriage law upon the procreative understanding of marriage?

Start your day with Public Discourse

Sign up and get our daily essays sent straight to your inbox.

The most important moral truth about the family is the radical equality and mutuality at the heart of family relationships, which relationships have an unbreakable foundation in the way children come to be within marriage. When the spouses’ marital acts bear the fruit of children, the children are perceptively called (in law) “issue of the marriage.” For children embody in a unique way their parents’ union. Just as the married couple is often referred to as two-in-one-flesh, so too each of their children is the two-of-them-in-the-one-flesh. Each child just is their union, extended into time and space, and thus into human history and into the whole human community. The parents can see in each of their children an unsurpassable reflection of them as a unity, that is, of their identity not as Jack and Joan but as the two-of-them-as-one flesh—literally.

Because all the married couple’s children come to be in and through the same act—separated only by time and perhaps by space—each child is equally and wholly the image of his or her parents’ unique union. All the children are, one compared to the others, equally and wholly the offspring of the same parents. Mother and father are equally and wholly parents of each child, in whom they see (literally) so many unique (but nonetheless identical, in the way just described) expressions of their own union. The siblings’ family identity is just that: a matter of identity. In an extended but profound sense, all of the couple’s children are (as it were) twins.

This matrix of familial equality, mutuality, and common identity is the wellspring and ground of love, duty, loyalty, care-giving—the whole moral culture of family life. The lifelong and unbreakable chords of fealty and relatedness which family members possess, one for the others, and which even distance and alienation never quite erase, depend upon it.

This biological common core is reflected all across our everyday lives. We see it in action every time someone announces the resolve to re-connect with his or her long-gone father. We see it in the face of every person who is reunited with a sibling long separated. We see it in our language. One to whom we become especially close is “like a brother” to us. The aunt or even an unrelated family friend who raised us is like “a mother,” and may even be called “mommy.” We see it in perhaps the most arresting image delivered to humankind, that of God as “Abba,” “our Father.”

The sublime equality and mutuality endemic to the family is neither mysterious nor dreamily metaphysical, and it surely is not so speculative as to be somehow beyond the state’s ken. It is no more subtle or beyond the state’s concern than is the correct judgment that the factor of equality of marital friendship lies at, or very near, the heart of the state’s legitimate judgment that polygamy is not supportable, even to the point of making criminal a person’s attempts (indeed, rendering their acts merely attempts) at plural marriage.

We can now see why the law has recognized marriage as morally normative for having children. The law has also recognized that marriage is normative for sexual activity, precisely in order to protect and preserve the valuable relationships that constitute the family. Incest is forbidden to protect the sibling relationship from ruin by sexual attraction and activity, and to forestall the grotesque prospect of there someday being “issue” of the issue. Legal bans on marriage within certain degrees of consanguinity enforce this taboo: if there is no possibility of sibling marriage (for example) then there is much less prospect of sibling sexual attraction and activity. Adultery is forbidden (though such prohibitions are not, as a rule, enforced) to promote the fidelity that defines spousal love. Fornication has historically been forbidden as a crime against marriage, and not for the sake of meddlesome moral paternalism, but because fornication breeds illegitimacy.

The metaphysical and philosophical structure of the family is a standing moral corrective for the cultural (and legal) distortions of the family which are all too familiar to students of history and current events: subordination of wives to husbands; parents’ treatment of children as extensions of their own plans and desires, almost as their property; children’s indifference to their parents who vouchsafed them life and whose marriage they (the children) embody. Recovering and burnishing the truth about the family is the sure first step towards genuine reform of family practices.