On Friday, when the United States Supreme Court recognized a constitutional right to marriage, it turned to history to explain its decision. “The history of marriage as a union between two persons of the opposite sex marks the beginning of these cases,” wrote Justice Anthony Kennedy, on behalf of the 5-4 majority. “To the respondents, it would demean a timeless institution if marriage were extended to same-sex couples.” But, he added, this view of marriage as timeless and unchanging was contradicted by an abundance of scholarly work. “The history of marriage is one of both continuity and change.”

When the Supreme Court met last April for oral arguments in Obergefell v. Hodges, the justices spent a lot of time talking about past. The question seemed to be—on whose side is history? Can a historical case be made for legalizing same-sex marriage, or would this be some kind of radical break with thousands of years of legal and cultural history of matrimony?

The history lesson began from nearly the first moment of oral arguments. Plaintiff’s attorney Mary Bonauto introduced her position, fielded a quick question from Justice Ginsburg, then argued that her client and a “whole class of people” were being denied the right to “join” in an “extensive government institution.” Chief Justice Roberts jumped in, “Well, you say join in the institution. The argument on the other side is that they're seeking to redefine the institution. Every definition that I looked up, prior to about a dozen years ago, defined marriage as unity between a man and a woman as husband and wife. Obviously, if you succeed, that core definition will no longer be operable."

Roberts was just the beginning. Justice Stephen Breyer, assumed to be sympathetic to the plaintiffs, also referenced thousands of years of marriage tradition that didn’t include same-sex couples. Anthony Kennedy and Antonin Scalia cited anthropological evidence for marriage customs, both ancient and contemporary. Justice Samuel Alito talked about ancient Greece and Plato. Bonauto was forced to concede, “I can’t speak to what was happening with the ancient philosophers.”

Fortunately, there are people who can speak to the ideas of ancient philosophers. I consulted Anise Strong, an assistant professor of history at Western Michigan University. Strong, the author of the forthcoming book Prostitutes and Matrons in the Roman World, studies gender and sexuality in the ancient world and the use of ancient history in the modern era. She notes that despite the approval by Plato and his social circle of “short-term, possibly unconsummated (‘platonic’) relationships between older man and teenage boys whose beards had not yet grown,” it’s true that ancient Greeks did not have gay marriage. Instead, they favored:

Closely endogamous marriages between uncles and nieces (and sometimes half-siblings), marriages in which women retained almost no property rights or independence and were regularly both physically segregated and violently abused, and a system in which marriage was designed explicitly to increase and safeguard the property of closely related men while encouraging the production of definitely legitimate male heirs to those men through tightly restricting access to their wives.

Strong also notes that across the great sweep of marriage across human history, societies have often embraced polygyny.

In other words, given that most Americans would find abhorrent the types of marriages of which ancient Athens approved, why should they care whether they would have disapproved of gay marriage? The history of ancient marriage is fascinating and important, but by citing Plato’s stance on gay marriage, the conservative justices are snatching out the one bit that’s useful to them, ignoring the broader cultural context. It’s using history to confirm biases.

In fact, when you really dig into the history of marriage, the only consistent feature is change. My own professional group, the American Historical Association, filed an amicus brief that leveraged the combined expertise of twenty historians of marriage. The AHA brief used examples drawn largely from American history to show that marriage has never been solely about procreation, with issues like property management taking center stage. Moreover, Ruth Karras, author of Unmarriages, told me in an interview that marriage has almost never been about joining one man and one woman, but instead about “two families.” In that sense, same-sex couples looking for equal protection under the law with respect to healthcare and property rights are pretty consistent with “traditional marriage.”

That is, if there even is such a thing as “traditional marriage.” Karras began studying the multiple forms of medieval marriage—or at least the socially-accepted and often semi-legal long-term forms of relationships—because of her frustration with the idea that, “there was some sort of time that we could go back and look at where marriage was this perfect ideal between a man and woman for purposes of reproduction or creating family. The Middle Ages clearly haunts that formulation.” In fact, Karras continued, for many medieval people, “traditional marriage didn't even exist. Yes, for aristocrats there was this system, but it's really not very possible to know much about how people without any money formed and possibly didn’t form their marriages. People seem to have this idea that until the 1960s in America, everybody was pro-marriage—in fact, in the Middle Ages a lot of people lived in other kinds of relations besides what was recognized formally as marriage.”

Karras likes to use “clerical marriage” as a good example of how social institutions change over time. During the later Middle Ages, the church refused to let priests marry. And yet, clerical marriages existed all over medieval Europe, and were often recognized by neighbors and local communities, although they had no legal standing, and non-birth parents had no rights over any offspring. She suggested this was a good analogy for the status of same-sex marriage in America over the last few decades, though it’s obviously rapidly becoming more legitimate. Today, of course, thanks to Protestantism and Orthodox Christianity, clerical marriage is more common than clerical celibacy among Christian populations, and even half of all Catholics worldwide think priests should be able to marry. “This kind of union, [once] grudgingly tolerated and informally recognized, is now formally recognized and tolerated,” Karras said.

Today, married status still confers major legal rights for partners, much as it did in past eras. But even that point of consistency indicates change. These legal benefits are adjacent to the real cultural center of marriage today—love. While the link between marriage and love dates back far into the recesses of history and literature, it was often marginal; the legal forms, particularly those affecting property, came first. Now, thanks to the popularization of romantic marriage in the 19th century, that relationship has flipped. In the words of historian Stephanie Coontz, “Love conquered marriage.”

On Friday, Kennedy cited the work of Coontz, and other historians, as he endorsed the continued evolution of marriage as an institution:

As women gained legal, political, and property rights, and as society began to understand that women have their own equal dignity, the law of coverture was abandoned...These and other developments in the institution of marriage over the past centuries were not mere superficial changes. Rather, they worked deep transformations in its structure, affecting aspects of marriage long viewed by many as essential…These new insights have strengthened, not weakened, the institution of marriage.

Decades of careful scholarship showing that the most constant element of marriage is change allowed the Court to decide that far from undermining the institution of marriage, some changes can actually strengthen it. I suspect that will be the case here.