Listening to a discussion about same-sex marriage can be like watching two ships pass in the night. Proceeding from different but all-too-often-unarticulated conceptions of what marriage is and why it matters for society, advocates on both sides tend to lock into patterns of reasoning that seem impervious to opposing arguments.

Proponents of redefining marriage to include same-sex couples point to the lack of fair legal accommodations for those who choose to live together and commingle their assets but who are not legally married. Exhibit A is the current Supreme Court case regarding the constitutionality of the Defense of Marriage Act (DOMA), which arose because Edith Windsor was required to forfeit over $360,000 in federal estate taxes upon the death of her lesbian partner. To many this seems unjust, especially when assets inherited from a legally married spouse are exempt from the estate tax altogether.

Advocates of the conjugal definition of marriage point to the tremendous impact on a child’s wellbeing of the circumstances of his conception. Decades of social science have confirmed that, all else being equal, children fare best when raised by their biological parents in a stable home. Yet children cannot barter for the privilege of being born into such circumstances; and so, as is the case with other externalities, it falls to higher rungs of civil society to promote norms of long-term commitment, comprehensive sharing of life, and sexual exclusivity among those who engage in the kind of activity that can result in conception.

Part of what can make the marriage debate seem so intractable is that both these aims are eminently reasonable as starting points for public policy. Adults who choose to share a life together do deserve fair treatment under the law, and children do deserve the benefits afforded by a loving mother and father in a stable home. Where those who call for the redefinition of marriage go wrong, I will argue, is in proposing to achieve both aims through a single policy instrument, thereby setting them in competition with each other. Because they allow space for policies that fit the contours of both aims, advocates of the conjugal definition of marriage are able to offer the better solution.

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Historically, societies have addressed the problem of child-rearing by institutionalizing, in the law as well as civil society, the integration of conjugal intercourse with long-term commitment, comprehensive sharing of life, and, in at least some societies, sexual exclusivity. Up until recently, the term “marriage” referred unambiguously to this institution.

In our society, this same institution also became associated with certain privileges, including an exemption from the estate tax. The problem, according to advocates of redefining marriage, is that it seems unjust not to grant some of these privileges more broadly.

Just how broadly, of course, may vary from privilege to privilege, and in some cases may be a matter of dispute. Some, for example, argue that we should abolish the estate tax altogether (I would be inclined to agree). What is important here is the general claim that at least some of the privileges currently associated with marriage fall under the heading of what is owed in justice to (at least) all who cohabit and commingle their assets.

Suppose for the sake of argument that this claim is true. If so, proponents of redefining marriage are right to challenge the status quo. The policy aim of guaranteeing fair treatment under the law to those who cohabit and commingle their assets applies to a population that is broader than the population to which the aim of integrating conjugal intercourse with long-term commitment, comprehensive sharing of life, and sexual exclusivity applies. As a result, a policy instrument tailored to achieve the second aim is doomed to be too narrow as an instrument for the first.

What proponents of redefining marriage miss is the fact that this problem will always remain as long as a single policy instrument is tasked with achieving both ends, for the simple reason that it cannot be tailored to both simultaneously. This is why simply expanding the scope of marriage to include same-sex couples is an inadequate solution. Far from being fitted to both policy aims, marriage redefined in this way would be fitted to neither: It would be at once too narrow and too broad.

It would be too narrow because it would continue to exclude polyamorous groups, as well as couples in a platonic relationship—bachelor brothers, say—who live together, care for each other, and share property. Yet who would say that the fairness of the estate tax levied on Edith Windsor hinges on whether she engaged in sexual acts with her longtime friend and roommate?

It would be too broad because same-sex couples cannot engage in the kind of activity that can result in conception, and hence there is no need for social norms that integrate sexual activity between persons of the same sex with an arrangement suitable for raising a child.

Indeed, redefining marriage would likely hamper its capacity to achieve the child-centric need at stake in the debate. Precisely insofar as they persuade us to conceptualize marriage in terms of shared living generally, proponents of redefining marriage will have rendered it incapable of shaping attitudes toward any particular kind of shared living. Yet to protect the wellbeing of children, the institution of marriage must set apart the particular kind of shared living best suited to child-rearing from the range of alternatives so as to promote it as an ideal among those who choose to engage in conjugal intercourse.

The best response to the two needs at stake in the marriage debate is to acknowledge that both are important and craft a solution that fits the contours of each. This does not require defining out of existence the only legal category whose purpose is precisely to integrate the kind of act that can result in conception with the kind of environment best suited for a child’s development. Instead, it merely requires disconnecting from marriage any privileges currently associated with it that are in fact owed in justice to a population broader than those who can become biological mothers and fathers.

In this way, not only could marriage continue to play its normative role in addressing the second policy aim at stake in the debate, but any injustice in tax policy, or policy regarding hospital visitation, could be eliminated for everyone, not just romantically involved same-sex couples.

The truth is that each of these policy needs is too important to be given short shrift. Reinforcement of the conjugal definition of marriage in the law combined with comprehensive reform in areas such as the estate tax would address both. Unfortunately, redefining marriage in a way that eliminates its child-centric focus, combined with denying fair treatment to those whom the new definition continues to exclude, would address neither.

Charles F. Capps is a management consultant based in Chicago, IL. A graduate of Stanford University, he will begin a joint JD/PhD program at the University of Chicago this fall.