An anti same-sex marriage demonstrator stands in front of the US Supreme Court in Washington, DC, June 18, 2015, awaiting the court's landmark decision. (Jim Watson/AFP/Getty Images)

William N. Eskridge Jr. is professor at the Yale Law School. He co-authored an amicus brief in the case Obergefell v. Hodges.

The Supreme Court is expected to decide the fate of marriage equality for same-sex couples by the end of the month. If the court requires the recognition of marriages between same-sex couples, the main criticism from those who object to the ruling will undoubtedly be that the unelected justices have “redefined” marriage contrary to the male-female definition accepted for “millennia.”

Indeed, the redefinition concern jump-started the April 28 oral arguments in Obergefell v. Hodges. Chief Justice John G. Roberts Jr. noted that every dictionary he checked that was published “prior to about a dozen years ago” defined marriage as between one man and one woman. Thus, if the court accepted the constitutional challenge, would it not be redefining and perhaps undermining marriage?

Justice Anthony Kennedy said: “This definition has been with us for millennia. It’s very difficult for the court to say, oh well, we know better.” Justice Samuel Alito asked: “How do you account for the fact that, as far as I’m aware, until the end of the 20th century, there never was a nation or a culture that recognized marriage between two people of the same sex?”

All of the justices and counsel addressing this point accepted the premise that no culture had ever recognized same-sex marriage. That premise is incorrect.

First- and second-century historians Suetonius and Tacitus (disapprovingly) documented official same-sex marriages in imperial Rome. Some modern historians have found plausible evidence of such marriages among Egyptians, Canaanites and Hittites and on islands in ancient Greece. So it is not right to say that the Western tradition had never entertained marriages between people of the same sex until the 20th century.

The evidence is overwhelming for non-Western cultures. In their 1951 book “Patterns of Sexual Behavior,” anthropologists Clellan Ford and Frank Beach surveyed 191 world cultures and found many examples of same-sex intimacy occurring “within the framework of courtship and marriage.” They were mainly referring to “berdache” marriages, in which a man would marry another man who performed domestic duties or a woman would marry a woman who worked outside the home. Researchers have demonstrated that a majority of Native American tribes (as well as many tribal people elsewhere in the world) have recognized such marriages at points in their histories.

Anthropologists have also documented the phenomena of “woman marriage” in African societies, in which a wealthy woman marries another woman and then secures her impregnation, thereby generating heirs. Anthropologist Denise O’Brien reports that such marriages have been recognized in more than 30 African cultures.

There are other examples (some more equivocal), but these show that there has been no universal definition of marriage that excludes same-sex couples. What is the point of this history?

One lesson is that “marriage” is an evolving, socially adaptive institution. In 1950, anthropologists defined “marriage” as a potentially procreative union of one man and one woman. But in the next 20 years, undisputed evidence of woman marriages, berdache marriages and other same-sex unions across dozens of cultures upended that definition. By the 1970s, anthropologists had settled on an understanding of marriage as a social institution serving a variety of purposes — not just procreation and inheritance, but also personal relationships and alliances.

In a sense, anthropologists “redefined” marriage, but a second lesson is this: They were professionally compelled to redefine the term once they learned that such institutions were in fact more pluralistic than previously thought. Far from imposing their own definition, they were recognizing a “redefinition” that better reflected the world.

We can return to the chief justice’s question and consider a third lesson. Traditional marriage law in this country was one man, one woman because its focus was state channeling of procreative intercourse. Thus, states such as Ohio (Jim Obergefell’s state) previously criminalized sex outside marriage, denied rights to illegitimate children, made wives legally subservient to their husbands and made it hard to divorce. These rules made it more likely that children would be born and raised in a marital environment.

Today, Ohio places no barrier to consensual sex outside marriage, treats nonmarital children and wives as equal citizens and allows easy divorce. Ohio has redefined the fundamental nature of marriage to accommodate almost all adults who love one another, including couples who have no intention or capacity to procreate.

About the only group left out of this redefined institution is gay people. Unless the court wants to redefine the constitutional requirement that states provide everyone the “equal protection of the laws,” that exclusion is a violation of the 14th Amendment.