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With the Supreme Court likely to hand down its decision on same-sex marriage shortly, let's consider the argument made by conservatives that seems to weigh most heavily with the public. Traditionalists warn against a "slippery slope" from gay rights to more radical change that would include legal recognition of polygamy. According to conservatives, those who want to redefine marriage to include same-sex couples cannot explain why it should stop there. At the Supreme Court's oral argument Justice Samuel Alito invoked not only polygamous marriage but also the caring relationship of a brother and sister who reside together. Assuming consent and mutual commitment, why not let them all wed?

If constitutional principles and reasoned reflection cannot help sort through the nettlesome issues related to marriage in its potential varieties, the Supreme Court may surprise us and refuse for now to recognize a constitutional right to same-sex marriage.

The key to resolving these questions lies in the Constitution's core commitment to securing equal liberty for all. If we turn from academic and judicial speculation about plural marriage to historical experience, it becomes clear that polygamy as a lived social form is at odds with equality. In societies where it has been practiced, polygamy has nearly always taken the form of one husband with multiple wives rather than one wife and multiple husbands. The husband and father is the hub. In this respect, polygamy is a patriarchal and hierarchical union-a system of male headship-that is fundamentally at odds with our constitutional values.

Indeed, wherever women have become equal, monogamy reigns. In no modern egalitarian society is there any broad social movement in favor of plural marriage. The contrast could not be more striking:

Polygamy and the equal rights of women and sexual minorities are on opposite historical trajectories.

Instead of a slippery slope toward polygamy, there is a historical slope in the opposition direction-toward monogamy, gender equality, and gay rights.

There is also another way in which polygamy conflicts with the interests in equal liberty. Plural marriage for the privileged means that large numbers of lower-status men will be unable to find spouses. And that excess of single males with no prospects of marriage has other ramifications. Single men are much more prone to substance abuse, violence, and risky behavior. Complex families composed of multiple wives and half siblings are also prone to greater jealousy, conflict, and violence. Allowing high-status males to satisfy their desire for multiple spouses thereby leads to greater violence in the home and in society. Monogamy, in contrast, gives men an incentive to invest their surplus resources in the education of their children, rather than acquiring additional wives. Monogamy is thus associated with greater cooperation, fairness, and progress.

During oral argument on the same-sex marriage case, Justice Ruth Bader Ginsburg and the attorneys defending marriage equality pointed out the relationship of same-sex marriage to equality. Same-sex unions fit comfortably with what marriage has become: an "egalitarian" relationship of two people committed to loving and caring for one another through all of life's trials, and also for children who become part of their family. Same-sex couples want access to the legal recognition, benefits, and obligations that help support this distinctive and vital relationship.

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But if marriage is extended to same-sex couples, how can other "consenting adults" be excluded? What about brothers and sisters, grandmothers and grandsons, and friends who live together in mutually supportive but non-romantic caring relationships? Such relationships have great value and our law and policy should do more to support them in appropriate ways. In fact, we already do allow for such support, as when unmarried siblings or friends delegate to one another powers of attorney in medical decision making.

Such relationships, however, are not marital. The spousal bond includes a sexual dimension that would be deeply destructive if introduced into other familial relations. The distinctive forms of love and care that characterize healthy family relations among parents and children depend on the exclusion of sexual relations.

Marriage has a deep and distinctive meaning and resonance. Same-sex couples seek access to the social meanings and norms, the obligations and benefits that surround marriage as an egalitarian institution.

The law of monogamous marriage provides a general framework that enables us to order our personal and social lives in ways that are profoundly valuable. It merits special recognition in our law and culture. Monogamous marriage reflects and reinforces our basic constitutional values of equality and fair opportunity. Same-sex marriage makes the institution better, fairer, and truer to our common ideals.