For supporters of gay marriage, the fears were stoked by Justice Anthony Kennedy — long considered at the vanguard of gay rights on the court — who came out of the gate early with questions about the dangers of upending the long tradition of limiting marriage to opposite-sex couples.

“The word that keeps coming back to me in this case is: millennia,” Kennedy said, referring to how long civilized society has conceived of marriage as a bond between one man and one woman. “This definition has been with us for millennia. And it’s very difficult for us to say, ‘Oh, well, we know better.’”

The scare for opponents of gay marriage came from Chief Justice John Roberts, who suggested the case might be resolved as a simple matter of sex discrimination — without even having to wade into the issue of how laws that affect gays should be treated by the courts.

“I’m not sure it’s necessary to get into sexual orientation to resolve the case. I mean, if Sue loves Joe and Tom loves Joe, Sue can marry him and Tom can’t. And the difference is based upon their different sex,” the chief justice asked. “Why isn’t that a straightforward question of sexual discrimination?”

Later in the arguments, as lawyers focused on the question of whether states had to recognize marriages performed in other states, Roberts again seemed to wander off what many considered his ideological reservation. He asked a lawyer for the state of Tennessee when, apart from gay marriages, that state last refused to recognize a marriage conducted in another state.

The answer was 1970, but Roberts’ question seemed to reflect a concern that states denying recognition to other states’ same-sex marriage would be disrupting the usual respect states show each other.

Moments later, the chief justice seemed to dismiss the notion that accepting same-sex marriages from other states would make it harder for states to deal with domestic disputes, divorces, child custody and the like. “It seems to me the question of how you apply domestic relations law is pretty straightforward,” Roberts said.

It was unclear whether Kennedy and Roberts’ intellectual wandering was enough to signal that many observers were wrong to expect the court to rule narrowly in favor of a federal constitutional right to same-sex marriage.

Both justices’ unexpected arguments could be feints or reflect the kind of devil’s advocate questioning intended to put the arguing lawyers through their paces. And at other points in the arguments, the two men returned to more expected lines of inquiry. Roberts, for example, fretted that forcing recognition of same-sex marriages could disrespect the democratic process. Kennedy, meanwhile, said same-sex couples were seeking the “dignity” of being recognized as married.

During two and a half hours of robust questioning, most of the other justices seemed to stick to their ideological corners. Justice Clarence Thomas, as usual, remained silent.

The court’s usual decorum was shattered about 30 minutes into Tuesday’s arguments, just as the lead-off lawyer in favor of same-sex marriage rights, Mary Bonauto of Gay and Lesbian Advocates and Defenders, wound up her arguments.

“The bible teaches if you support [same-sex-marriage] you will burn in hell. It’s an abomination,” a man in the back of the gallery bellowed. He continued screaming as he was led out by court police and his somewhat-muffled shouts from elsewhere in the building could be heard clearly in the courtroom for several minutes thereafter.

“It was rather refreshing, actually,” Justice Antonin Scalia remarked, without indicating whether he was speaking to the man’s views or the break from the monotony.

The same-sex marriage issue returned to the court Tuesday two years after the justices handed gay-rights advocates an incomplete victory by striking down a key part of the federal Defense of Marriage Act, but punting on the question of same-sex marriage nationally by dismissing a case from California on technical grounds.

Kennedy wrote the 5-4 majority opinion in that decision, in which the court held it unconstitutional to deny federal benefits to same-sex couples legally married in states that permit the practice.

Despite a wave of lower-court rulings in favor of same-sex marriage last year, the justices kept the issue at bay until January, when they agreed to hear the four cases argued Tuesday involving challenges to same-sex marriage bans in Kentucky, Michigan, Ohio and Tennessee.

A decision from the justices is expected by late June.