At stake in these cases is not who is and is not eligible to marry but what marriage is. Today’s debates offer rival answers to that question, two substantive marriage definitions. This Court’s task is not to judge the desirability of the Defense of Marriage Act (“DOMA”) and California Proposition 8’s definition, but only to decide whether citizens and legislators may embody in law the belief in marriage as a conjugal union, as they have historically done.There are excellent reasons from philosophy and social science to think that marriage is a conjugal relationship — the type of union that only a man and woman can form — rather than just the sort of emotional union that any two (or more) adults can form. And recognizing marriage as such serves important public interests.Indeed, the most important free community, on which all others depend, is the marriage-based family. But to thrive, it requires a supporting framework of understandings and expectations. A main purpose of marriage law in any society is to promote such a culture. Sound marriage policy therefore serves the common good (especially the institutions of civil society) and helps keep government limited.Redefining civil marriage can cause corresponding social harms. It weakens the rational foundation (and hence social practice) of the stabilizing marital norms on which social order depends: norms such as permanence, exclusivity, monogamy. Conferring benefits on same-sex relationships itself does not do this, but redefining marriage in the public mind does. And undermining the norms of marriage will in turn damage the many cultural and political goods that draw the law into the marriage business. We list them in summary form here.Real marital fulfillment. To form a true marriage, one must freely choose it, which requires at least a rough idea of what it actually is. Redefining marriage will harm people (especially future generations) by distorting their idea of what marriage is. It will teach that marriage is essentially about emotional fulfillment, without any inherent connections to bodily union or procreation and family life. As people internalize this view, their ability to realize genuine marital union will diminish.Child and spousal well-being. Marriage tends to make spouses healthier, happier and wealthier. And it is marriage itself — conjugal marriage — that does this, especially through its distinctive norms of permanence, exclusivity and orientation to family life. As the state’s redefinition of marriage makes these norms harder to understand, cherish, justify and live by, spouses will benefit less from the advantages of stability.Moreover, if marriage is redefined, no civil institution will reinforce the notion that men and women tend to bring different gifts to child-rearing. In all these ways, redefinition will lower the pressures and incentives for men and women to stay with their spouses and children, or for couples to marry before conceiving. This would harm children’s development as children do best when raised by their married biological mother and father. The welfare and correctional state will have to expand to fill the developmental vacuum.Leading LGBT scholars and activists increasingly agree that redefining marriage would undermine its norms.Religious liberty. If marriage understood as the union of man and woman comes to be seen as irrational (“bigotry”), freedom to express and live by this idea will be eroded. Individuals and institutions who espouse the conjugal view have been denied government licenses, or educational and professional opportunities, for living by (or even publicizing) their views. The consequences for observant Christians, Jews, Muslims and others are clear.Moreover, none of these harms is caused by recognizing infertile (opposite-sex) marriages, which cohere with the conjugal view. And finally, enshrining this view of marriage in law is fully consistent with this Court’s ruling in Lawrence v. Texas.Because there are good reasons for citizens and lawmakers to conclude that marriage is a union of man and woman — even before considering the harms that redefinition might bring — this Court should uphold DOMA and Proposition 8 as constitutional exercises of policy-making power by Congress and the citizens of California, respectively.