This is an important set of excerpts from the book, What is Marriage?, and is linked to my Cumulative Case. I highly recommend getting the book and reading chapters three and four, you can also follow up on the many references to the quotes I did not include below:

Against this, some on the libertarian Right say that mar­riage has no public value, and call for the state to get out of the marriage business altogether. Voices on the Left say that marriage has no distinctive public value; they say the state may work it like clay, remaking marriage to fit our preferences. Here we show where both go wrong.

[….]

First, as we have seen by reflection that procreation uniquely extends and perfects marriage (see chapter 2), so the best available social science suggests that children tend to do best when reared by their married mother and father. Studies that control for other factors, including poverty and even genetics, suggest that children reared in intact homes do best on the following indices:

Educational achievement: literacy and graduation rates

Emotional health: rates of anxiety, depression, substance abuse, and suicide

Familial and sexual development: strong sense of identity, timing of onset of puberty, rates of teen and

out-of-wedlock pregnancy, and rates of sexual abuse

Child and adult behavior: rates of aggression, attention deficit disorder, delinquency, and incarceration

Consider the conclusions of the left-leaning research institution Child Trends:

[R]esearch clearly demonstrates that family structure matters for children, and the family structure that helps children the most is a family headed by two biological parents in a low-conflict marriage. Children in single-parent families, children born to unmarried mothers, and children in stepfamilies or cohabiting relationships face higher risks of poor outcomes. . . . There is thus value for children in promoting strong, stable marriages between biological parents. . . . [Fit is not simply the presence of two parents, . . . but the presence of two biological par­ents that seems to support children's development.

According to another study, in the Journal of Marriage and Family, "[t]he advantage of marriage appears to exist primarily when the child is the biological offspring of both parents.” Recent literature reviews conducted by the Brookings Institu­tion, the Woodrow Wilson School of Public and International Affairs at Princeton University, the Center for Law and Social Policy, and the Institute for American Values corroborate the importance of intact households for children.♦

Single-motherhood, cohabitation, joint custody after di­vorce, and stepparenting have all been reliably studied, and the result is clear: Children tend to fare worse under every one of these alternatives to married biological parenting. To make marriages more stable is to give more children the best chance to become upright and productive members of society. Note the importance of the link between marriage and children in both stages of our argument: just as it provides a powerful reason to hold the conjugal view of marriage, so it provides the central reason to make marriage a matter of public concern.

But this link is no idiosyncrasy of our view. It is amply con­firmed in our law. Long before same-sex civil marriages were envisioned, courts declared that marriage “is the foundation of the family and of society, without which there would be nei­ther civilization nor progress.” They recalled that “virtually every Supreme court case recognizing as fundamental the right to marry indicates as the basis for the conclusion the institu­tion’s inextricable link to procreation.” In their account, not just ours, “the first purpose of matrimony, by the laws of nature and society, is procreation”; “the procreation of children un­der the shield and sanction of the law” is one of the “two princi­pal ends of marriage.” In fact, “marriage exists as a protected legal institution primarily because of societal values associated with the propagation of the human race.” Examples can be multiplied ad nauseam.

A second public benefit of marriage is that it tends to help spouses financially, emotionally, physically, and socially. As the late University of Virginia sociologist Steven Nock showed, it is not that people who are better off are most likely to marry, but that marriage makes people better off. More than signal maturity, marriage can promote it. Thus men, after their wed­ding, tend to spend more time at work, less time at bars, more time at religious gatherings, less time in jail, and more time with family.

The shape of marriage as a permanent and exclusive union ordered to family life helps explain these benefits. Permanently committed to a relationship whose norms are shaped by its apt­ness for family life, husbands and wives gain emotional insur­ance against life’s temporary setbacks. Exclusively committed, they leave the sexual marketplace and thus escape its heightened risks. Dedicated to their children and each other, they enjoy the benefits of a sharpened sense of purpose. More vigorously sow­ing in work, they reap more abundantly its fruits. So the state’s interest in productivity and social order creates an interest in marriage.

[….]

MAKING MOTHER OR FATHER SUPERFLUOUS

Conjugal marriage laws reinforce the idea that the union of husband and wife is, on the whole, the most appropriate envi­ronment for rearing children—an ideal supported by the best available social science.† Recognizing same-sex relationships as marriages would legally abolish that ideal. No civil institution would reinforce the notion that men and women typically have different strengths as parents; that boys and girls tend to benefit from fathers and mothers in different ways.

To the extent that some continued to see marriage as apt for family life, they would come to think—indeed, our law, public schools, and media would teach them, and variously penalize them for denying—that it matters not, even as a rule, whether children are reared by both their mother and their father, or by a parent of each sex at all. But as the connection between mar­riage and parenting is obscured, as we think it would be eventu­ally, no arrangement would be proposed as ideal.

And here is the central problem with either result: it would diminish the social pressures and incentives for husbands to remain with their wives and children, or for men and women having children to marry first. Yet the resulting arrangements—parenting by divorced or single parents, or cohabiting couples —are demonstrably worse for children, as we have seen in chap­ter 3. So even if it turned out that studies showed no differences between same- and opposite-sex parenting, redefining marriage would undermine marital stability in ways that we know do hurt children.

That said, in addition to the data on child outcomes sum­marized in chapter 3, there is significant evidence that moth­ers and fathers have different parenting strengths—that their respective absences impede child development in different ways. Girls, for example, are likelier to suffer sexual abuse and to have children as teenagers and out of wedlock if they do not grow up with their father. For their part, boys reared without their father tend to have much higher rates of aggression, de­linquency, and incarceration. As Rutgers University sociolo­gist David Popenoe concludes, “The burden of social science evidence supports the idea that gender-differentiated parenting is important for human development and that the contribution of fathers to childrearing is unique and irreplaceable.” He con­tinues: “[W]e should disavow the notion that ‘mommies can make good daddies,’ just as we should disavow the popular notion . . . that ‘daddies can make good mommies.’ . . . The two sexes are different to the core, and each is necessary—culturally and biologically—for the optimal development of a human being.” In a summary of the relevant science, Univer­sity of Virginia sociologist W. Bradford Wilcox finds much the same:

Let me now conclude our review of the social scientific lit­erature on sex and parenting by spelling out what should be obvious to all. The best psychological, sociological, and biological research to date now suggests that—on average—men and women bring different gifts to the parenting enterprise, that children benefit from having parents with distinct parenting styles, and that family breakdown poses a serious threat to children and to the societies in which they live.

Of course, the question of which arrangements our policies should privilege is normative [should be based on natures/natural conditions]….

♦ Note that for a relationship to be ordered to procreation in this principled and empirically manifested way, sexual orientation is not a disqualifier. The union of a husband and wife hears this connection to children even if, say, the husband is also attracted to men. What is necessary is rather sexual complementarity—which two men lack even if they are attracted only to women. It is not individuals who are singled out—as being less capable of affectionate and responsible parenting, or anything else. What are instead favored as bearing a special and valuable link to childrearing are certain arrangements and the acts that complete or embody them—to which, to be sure, individuals are more or less inclined.

† The need for adoption (and its immense value) where the ideal is practically impossible is no argument for redefining civil marriage, a unified structure of incentives meant precisely to reinforce the ideal—to minimize the need for alternative, case-by-case provisions.