WASHINGTON -- Supreme Court justices revealed sharp and passionately held differences Tuesday as they confronted California’s Proposition 8 ban on gay marriages.

During an 80-minute argument that was unusually long and, at times, markedly heated, the back and forth between the court’s conservative and liberal wings foreshadowed difficult decisions to come. Tellingly, the frequent swing vote, Justice Anthony Kennedy, revealed some ambivalence.

In a positive sign for gay marriage supporters, Kennedy voiced strong sympathy for the approximately 40,000 California children who live with same-sex couples.

“They want their parents to have full recognition and full status,” Kennedy said, with evident feeling, adding that “the voice of these children is important in this case.”

But in a sign of how complicated the outcome might be, Kennedy and Justice Sonia Sotomayor mused aloud about whether the Supreme Court should have agreed to hear the case called Hollingsworth v. Perry at all. The court, if it chooses, has several possibilities for issuing either a very narrow decision or ducking the case altogether.

“The problem with this case is that you’re really asking . . . for us to go into uncharted waters,” Kennedy cautioned anti-Proposition 8 lawyer Theodore Olson.

With hundreds of demonstrators amassed outside, and prominent individuals – including Hollywood director Rob Reiner, “Milk” screenwriter Dustin Lance Black and California Lt. Gov. Gavin Newsom – gathered within, the justices were debating whether the Proposition 8 same-sex marriage ban violated constitutional guarantees of equal protection. Reiner helped found the American Foundation for Equal Rights, which organized opposition to Proposition 8.

Though California Attorney General Kamala Harris, who was also present, noted that “it’s a mistake to make a prediction about a justice’s state of mind based on a question,” some judicial inclinations seem apparent.

There seemed to be little to no support for an Obama administration proposal that would recognize a right to gay marriage in the eight states that, like California, ban gay marriage but recognize gay civil unions. A sweeping decision covering all 50 states didn’t leap out as an obvious solution, either, with Olson advising justices that they “could write a narrower decision.”

Chief Justice John Roberts Jr., along with fellow conservative Justices Samuel Alito and Antonin Scalia, appeared most sympathetic to the arguments of Proposition 8 supporters.

“Traditional marriage has been around for thousands of years,” Alito declared, while “same-sex marriage is very new . . . so there isn’t a lot of data about its effect. It may turn out to be a good thing; it may turn out not to be a good thing.”

His voice rising, Scalia further pressed Olson to explain “when did it become unconstitutional to prohibit gays from marrying?”

Proposition 8, Olson argued, “walls off gays and lesbians from marriage, the most important relation in life.”

Attorney Charles Cooper, the former Reagan administration official arguing in support of Proposition 8, stressed that recognizing same-sex marriages would “sever (marriage’s) abiding connection with its historic traditional procreative purposes.”

“Marriage itself is the institution that society has always used to regulate these heterosexual, procreative relationships,” Cooper said.

In turn, Justice Elena Kagan countered with the example of older couples who marry despite being past child-rearing age.

“There are lots of people that get married that can’t have children,” Justice Stephen Breyer added.

The justices divided their time between discussing whether Proposition 8 supporters had the “standing” to argue the case, since California officials refused to defend the initiative, and the measure’s underlying merits. The standing question might become an off-ramp for the case, short of a big decision. If the court decides that the Proposition 8 supporters lack standing, that kicks the case all the way back to the original decision by U.S. District Judge Vaughn Walker striking down the ballot measure. The legal consequences of that remain uncertain.

“If the issue is letting the states experiment and letting the society have more time to figure out its direction, why is taking a case now the answer?” Sotomayor asked rhetorically.

Most of the oral argument time, though, was spent on the merits.

San Francisco officials legalized gay marriage in February 2004. Several thousand same-sex couples were married before the state Supreme Court blocked the city’s action. The California Supreme Court subsequently recognized same-sex marriage rights in May 2008, after which about 18,000 same-sex couples were married.

In November 2008, the state’s voters approved Proposition 8, which amended the California Constitution to declare that “only marriage between a man and a woman is valid or recognized” in the state. The 9th U.S. Circuit Court of Appeals, in turn, struck down Proposition 8 on the grounds that it stripped California residents, without a legitimate justification, of a right that had previously been granted.

More than 110 reporters spilled over from the courtroom Tuesday into an adjacent hallway, many sitting behind pillars that made it impossible to see all the justices who were speaking. All told, some 400 lawyers, guests and members of the public attended, with some public visitors staying for only about five minutes before being rotated out.

Justice Clarence Thomas, as his custom, was the only high court member not to speak or ask a question.

A decision in the Proposition 8 case, as well as the Defense of Marriage Act case being heard Wednesday, is expected by the end of June.

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