In the 1860s New York lawyer and anthropologist Louis Henry Morgan attempted a systematic cross-cultural study of the institution of marriage. Morgan’s data were imperfect, but he was able to demonstrate that the record of human societies showed a startling diversity of socially approved forms of marriage. All societies had some form of regularized partnership, but no single standard human form could be identified. Generally, even within a society, there was a certain elasticity of marriage forms.

The most famous of these unions were the ones most foreign to Western Victorian society: marriage between a woman and several men; marriage between a man and several women; forms of “visiting” marriage, whereby a man might visit his wife but not live with her. As anthropologists assembled more reliable data, they found it difficult to produce a definition of human marriage that would hold true for all its socially legitimate forms.

Marriage generally functioned to provide a “legitimate” identity to children — a kind of “last name.” Yet, the structure of these arrangements was extraordinarily diverse: Biological paternity was not universally the basis of identity — as, indeed, it is not in the case of adoption in America. In many cases, the biological father (the Latin term is genitor) was distinct from the legal father (pater) produced by the marriage contract and ceremony. Alternatively, it could be the mother’s family and not the father that bestowed identity on a child.

As for sex, rarely if ever has marriage been able to restrict its varied practice to the relation of man and wife. In most cases, anthropologists agreed, what counted was that some socially approved form of marriage provided a secure place for the child in the social order.

But marriage has not been solely about children. In most societies known to us, everyone marries; it is an expected rite of passage and part of the normal life course of all adults. Only in post-classical Western societies do we find high numbers of unmarried people. Unlike other peoples, we consider marriage — however desirable or undesirable — optional.

Claude Levi-Strauss, the father of French structural anthropology, argued that it is only the “division of labor between the sexes that makes marriage indispensable.” It follows that if men and women are granted equal access to jobs of similar worth — as is often the case today — the meaning of marriage will change.

The cult of romantic love in a companionate marriage is a recent innovation in the history of marriage. While romantic passion has existed in all societies, only in a few has this unstable emotion been elaborated and intensified culturally and considered the basis for the social institution of marriage. Indeed, marriage has traditionally been more concerned with — and successful in — regulating property relations and determining lineage or inheritance rights than with confining passion and sexual behavior.

Marriage, in other words, is not only diverse across cultures but also dynamic and changing in America’s own history. We live in a pluralist society, where marriage is not the only form of union or of mutual care in our society. When individuals and groups can, under certain conditions, choose their patterns of self-expression — their intimacy, child-care arrangements, sexual practices, place of residence, partnership forms — there will be increased variability. The meaning of marriage — and the value of marriage — changes when it becomes one of several options in a society of self-determining individuals.

This said, it is not the case that “anything goes.” Every society favors forms of union that conform to its ethical standards and its needs.

Our society no longer approves of treating women as incompetent minors and the wards of their husbands within the structure of a patriarchal union. We do not approve, generally, of plural marriages — the basis of our disapproval being that they abrogate the rights of women and especially of young girls. We no longer generally feel that the sole function of women in society is to produce children and serve men as domestic labor. In other words, when we censure certain types of marriage, the basis on which we do so is our defense of individual human rights. This is our ethical standard.

Marriage is, then, foundational because it provides a recognized form of identity and security for children in society. Its function is not universally to produce children but to provide legitimate forms for their care. And marriage’s primary accomplishment is not to regulate sex (as a quick glance at American society would tell us). The institution survives despite infidelity, and sex does not by itself create marriage.

In addition, it is a system of exchange whereby families “give up” their own offspring to make new alliances with others, and to enter into broad networks of relationships, including and especially with one’s “enemies.” Without such arrangements, we would have a world of isolated, incestuous, biological clans — and endemic warfare.

What, then, about restriction of the legal bond of marriage to a man and a woman? Does marriage have to be heterosexual? The human record tells us otherwise. While the model of marriage is arguably heterosexual, the practice of marriage is not. In a broad spectrum of societies in Africa, for example, when a woman’s husband dies, she may take on his legal role in the family, and acquire a legal “wife” to help manage the domestic establishment. This role of wife is above all social, and not contingent on her sexual relations. These societies, which practice heterosexuality, take this woman-woman marriage as commonsensical; they recognize that above all marriage functions socially to extend and stabilize the network of care.

As for marriage as a legal institution, the ethnographic record makes clear that law expresses the dominant ethics of the group. Our history reflects the evolution of our values, and we as Americans are most proud of our deepening tradition of civil rights. To deny marriage to same-sex couples, as President Bush proposes, expresses a rejection of this civil rights tradition and a regression to a politics of exclusion.