Civil, substantive arguments on the nature and purpose of marriage can sometimes get lost in rancorous rhetorical crossfire over the definition of the institution. John Corvino and Maggie Gallagher’s exchange in their new book Debating Same-Sex Marriage is a welcome exception.

For Corvino, marriage establishes your “Number One Person,” that “special someone” whom “you will come home to at night, wake up with in the morning, and share life’s joys, sorrows, and challenges with.” But his presentation of marriage makes it seem more like companionship or best-friendship, and thus it is hard to see why the government is even in the marriage business.

Gallagher seeks to explain both the private and public meaning of marriage and thus why the state should rightly take an interest in marital unions. She explains that “marriage is a sexual union of male and female oriented toward connecting fathers to mothers and their children.” Marriage is a social institution that helps unite goods and persons whose fracture would result in great social cost:

The critical public or “civil” task of marriage is to regulate sexual relationships between men and women in order to reduce the likelihood that children (and their mothers, and society) will face the burdens of fatherlessness, and increase the likelihood that there will be a next generation that will be raised by their mothers and fathers in one family, where both parents are committed to each other and to their children.

Gallagher doesn’t say this to denigrate other relationships but to stress that not every loving, care-giving relationship is a marriage. Marriage has its distinct contours and norms in part because of its social function.

Gallagher fears that redefining marriage to include same-sex relationships will cause harm—not immediately, but over time, as it shapes culture. First, it will brand as bigots those who see differences between same- and opposite-sex relationships. Cultural stigma and legal penalties will await those who continue to argue that children need married moms and dads.

Second, she notes that the traditional norms of marriage—monogamy, sexual exclusivity, and permanence—make less sense once marriage is no longer bridging the gender divide and is severed from its orientation to procreation. Gallagher cites studies showing that people in same-sex relationships report less of an interest in and satisfaction from such norms, and she fears that those norms, instead of shaping same-sex relationships, will simply be further weakened, leading to more non-marital childbearing and more social strain.

Finally, Gallagher thinks that anyone who takes marriage seriously as a child-protecting institution should proceed with caution before redefining the institution and changing the cultural message that it sends. Before we embark on such a vast social experiment to benefit such a small population, she argues, the “burden of proof” should be on those who would redefine the institution to show that it wouldn’t obscure the central function of marriage and thus have a negative effect in the broader society.

Law and public policy can play a role in helping to strengthen the marriage culture. And Gallagher is clear that “stopping gay marriage is not victory, it is only a necessary step to the ultimate victory: the strengthening of a culture of marriage that successfully connects sex, love, children, and mothers and fathers.”

My full review of Debating Same-Sex Marriage appeared in the July 30 issue of National Review. Read it here.