By Michael Scherer
April 30, 2015

All Justice Anthony Kennedy needed to do was to start with a wink or a nod, maybe a stray phrase, to set the nation’s gay-rights advocates at ease.

For two decades, he has been the Supreme Court’s pivotal vote for same-sex couples, often tipping the balance of a divided bench. He wrote opinions that struck down bans on sodomy and ended legislated discrimination against homosexuals. In 2013 he forced the federal government to recognize same-sex spouses because to do otherwise “humiliates” their children. Now the court is confronting the next questions in the nation’s rapid cultural transformation: Are state bans on gay marriage constitutional? And if so, do all states still need to recognize out-of-state same-sex weddings?

But as arguments started on April 28, Kennedy was in no mood to show his cards. He began instead by discussing the rituals of the Kalahari people, an African tribe descended from ancient lineage, who never recognized same-sex unions as a form of marriage. His point was that marriage had until recently always been tied to childbearing. “This definition has been with us for millennia,” he told Mary Bonauto, the attorney for same-sex couples. “And it’s very difficult for the court to say, ‘Oh, well, we know better.'”

At a historic moment, history had become the issue. Justice Antonin Scalia pointed out that no society prior to the Netherlands in 2001 had ever permitted same-sex weddings. Justice Samuel Alito cited Plato, who approved of same-sex relationships but not same-sex marriage. “I can’t speak to what was happening with the ancient philosophers,” Bonauto responded.

It was an unexpected beginning to a case legal scholars have long handicapped as a likely win for gay-rights advocates, which could make same-sex marriage legal nationwide when a decision is delivered in June. John Bursch, Michigan’s former solicitor general, did his best to hold back that result, arguing that voters–not courts–should decide whether the institution should be defined around the genetic process of reproduction. “The state doesn’t care about your sexual orientation,” he said. “What the state cares about is that biological reality.”

To this, Donald Verrilli Jr., the Obama Administration’s attorney, offered a warning. Allowing the bans to stand would return the nation to a darker era, when many states had laws that barred interracial marriage, a practice the Supreme Court overturned in 1967. “You will have a minority of states in which gay couples will be relegated to demeaning, second-class status,” he said.

On this point, Kennedy seemed sympathetic. When Bursch claimed that granting marriage was unrelated to “bestowing or taking away dignity from anyone,” Kennedy objected. “I thought that was the whole purpose of marriage,” the crucial Justice said. “I’m puzzled.”

That same sense of equality had been at the heart of Kennedy’s past two rulings on same-sex relationships. He didn’t just strike down antisodomy laws, but he also declared that the effort to justify them “demeans the lives of homosexual persons.” In that case, Kennedy dispatched with a 17-year-old court precedent. Now he faces a bigger challenge: to weigh the dignity of America’s gays and lesbians against the burdens of millennia.

Contact us at editors@time.com.

This appears in the May 11, 2015 issue of TIME.

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