... Finally, the whole premise of marriage as an unchanging institution across thousands of years is a myth, as numerous people have repeatedly pointed out. An exchange between Justice Ginsburg and Mary Bonauto, the lawyer arguing for marriage equality, raised this point directly. A millennium ago, Ginsburg noted, marriage was “a dominant and a subordinate relationship” between a man and a woman, in which the man made all decisions and the woman was considered his property. Bonauto agreed. “Because of equality and changing social circumstances” over the last century, the way the law sees those gender differences within marriage have fallen away. As it’s now understood, Bonauto said, marriage “is a system in which committed, same-sex couples fit quite well.” In other words, it’s not gays and lesbians who are seeking to change marriage; the institution has always been changing, and given how the law currently views it, there’s no good reason to exclude gay couples.

In the dramatic 2010 trial challenging California’s marriage ban, Prop 8, the Harvard historian Nancy Cott explained how American marriage is a recent innovation, since most of the world at the time the United States was founded practiced polygamous marriages, not those of one man and one woman. Looking to history, she testified, clearly shows that marriage has long had many different purposes, including maintaining social order, forming stable households, distributing economic benefits, and cementing ties of affection. Procreation was just one among several, undercutting the narrow, biology-based arguments of the lawyers defending gay marriage bans.

Part of the reason the justices raised the issue of “millennia” of straight-only marriage was to suggest that animus, moral disapproval, and religious faith—all constitutionally impermissible rationales—are not the only reasons a state might opt to ban gay marriage. If all these cultures for all this time—even ancient Greece!—refused to allow gay marriage, there could be any number of rational bases for that position, goes the argument. As a result, its proponents conclude, the “democratic process” should be allowed to play out.

But that’s just another excuse for the justices not to do their job. ...