WASHINGTON — The Supreme Court dove into a historic debate on gay rights Tuesday that could soon lead to resumption of same-sex marriage in California, but the justices signaled they may not be ready for a major national ruling on whether America’s gays and lesbians have a right to marry.

The court’s first major examination of gay rights in 10 years continues Wednesday, when the justices will consider the federal law that prevents legally married gay couples from receiving a range of benefits afforded straight married people.

The issue before the court on Tuesday was more fundamental: Does the Constitution require that people be allowed to marry whom they choose, regardless of either partner’s gender? The fact that the question was in front of the Supreme Court at all was startling, given that no state recognized same-sex unions before 2003 and 40 states still don’t allow them.

There is no questioning the emotions the issue stirs. Demonstrators on both sides crowded the grounds outside the court, waving signs, sometimes chanting their feelings.

Inside, a skeptical Justice Samuel Alito cautioned against a broad ruling in favor of gay marriage precisely because the issue is so new.

“You want us to step in and render a decision based on an assessment of the effects of this institution which is newer than cellphones or the Internet? I mean, we do not have the ability to see the future,” Alito said.

Indeed, it was clear from the start of the 80-minute argument in a packed courtroom, that the justices, including some liberals who seemed open to gay marriage, had doubts about whether they should even be hearing the challenge to California’s Proposition 8, the state’s voter-approved gay marriage ban.

Justice Anthony Kennedy, the potentially decisive vote on a closely divided court, suggested the justices could dismiss the case with no ruling at all.

Such an outcome would almost certainly allow gay marriages to resume in California but would have no impact elsewhere.

There was no majority apparent for any particular outcome, and many doubts were expressed by justices about the arguments advanced by lawyers for the opponents of gay marriage in California, by the supporters and by the Obama administration, which is in favor of same-sex marriage rights. The administration’s entry into the case followed President Barack Obama’s declaration of support for gay marriage.

On the one hand, Kennedy acknowledged the recentness of same-sex unions, a point stressed repeatedly by Charles Cooper, the lawyer for the defenders of Proposition 8. Cooper said the court should uphold the ban as a valid expression of the people’s will and let the vigorous political debate over gay marriage continue.

But Kennedy pressed him also to address the interests of the estimated 40,000 children in California who have same-sex parents.

“They want their parents to have full recognition and full status. The voice of those children is important in this case, don’t you think?” Kennedy said.

Yet when Theodore Olson, the lawyer for two same-sex couples, urged the court to support such marriage rights everywhere, Kennedy feared such a ruling would push the court into “uncharted waters.” Olson said that the court similarly ventured into the unknown in 1967 when it struck down bans on interracial marriage in 16 states.

Kennedy challenged the accuracy of that comment, noting that other countries had had interracial marriages for hundreds of years.

The justice, whose vote usually decides the closest cases, also made clear he did not like the rationale of the federal appeals court that struck down Proposition 8, even though it cited earlier opinions in favor of gay rights that Kennedy had written.

That appeals court ruling applied only to California, where same-sex couples briefly had the right to marry before the state’s voters in November 2008 adopted Proposition 8, a constitutional amendment that defined marriage as the union of a man and a woman.

Several members of the court also were troubled by the Obama administration’s main contention that when states offer same-sex couples civil union rights of marriage, as California and eight other states do, they also must allow marriage. The other states are: Colorado, Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon and Rhode Island.

“So a state that has made considerable progress has to go all the way, but at least the government’s position is, if the state has done absolutely nothing at all, then it can do as it will,” Justice Ruth Bader Ginsburg said.

Chief Justice John Roberts questioned whether gay marriage proponents were arguing over a mere label. “Same-sex couples have every other right. It’s just about the label,” Roberts said.

In the California case, if the court wants to find an exit without making a decision about gay marriage, it has two basic options.

It could rule that the opponents have no right, or legal standing, to defend Proposition 8 in court. Such an outcome also would leave in place the trial court decision in favor of the two same-sex couples who sued for the right to marry. On a practical level, California officials probably would order county clerks across the state to begin issuing marriage licenses to gay and lesbian couples, although some more conservative counties might object.

Alternatively, the justices could determine that they should not have agreed to hear the case in the first place, as happens a couple of times a term on average. In that situation, the court issues a one-sentence order dismissing the case “as improvidently granted.” The effect of that would be to leave in place the appeals court ruling, which in the case of Proposition 8, applies only to California. The appeals court also voted to strike down the ban, but on somewhat different grounds than the trial court.

Both sides of the case were well represented outside the courthouse. Supporters of gay marriage came with homemade signs including ones that read “a more perfect union” and “love is love.”

Among the opponents was retired metal worker Mike Krzywonos, 57, of Pawtucket, R.I. He wore a button that read “marriage 1 man + 1 woman” and said his group represents the “silent majority.”

Same-sex marriage is legal in nine states and the District of Columbia. The states are Connecticut, Iowa, Maine, Maryland, Massachusetts, New Hampshire, New York, Vermont and Washington.

Thirty states ban same-sex marriage in their constitutions, while ten states bar them under state laws. New Mexico law is silent on the issue.

Polls have shown increasing support in the country for gay marriage. According to a Pew Research Center poll conducted in mid-March, 49 percent of Americans now favor allowing gays and lesbians to marry legally, with 44 percent opposed.

A good part of the give-and-take Tuesday concerned Cooper’s argument that the state has a legitimate interest in limiting marriage to heterosexuals since they have the unique ability to have children.

He and Justice Elena Kagan engaged in a lengthy, sometimes humorous, exchange on the topic.

If a state can use the ability to have children as a reason to prohibit same-sex marriage, what about couples over the age of 55? Kagan asked.

“Your Honor, even with respect to couples over the age of 55, it is very rare that both parties to the couple are infertile,” Cooper said.

Kagan cut in: “I can just assure you, if both the woman and the man are over the age of 55, there are not a lot of children coming out of that marriage.”

At another point, Justice Antonin Scalia, who has dissented in the court’s previous gay rights cases, invoked the well-being of children to bolster Cooper’s case.

“If you redefine marriage to include same-sex couples, you must permit adoption by same-sex couples, and there’s considerable disagreement among sociologists as to what the consequences of raising a child in a single-sex family, whether that is harmful to the child or not,” Scalia said.

The California case was argued 10 years to the day after the court took up a challenge to Texas’ anti-sodomy statute. That case ended with a forceful ruling prohibiting states from criminalizing sexual relations between consenting adults.

Kennedy was the author of the decision in Lawrence v. Texas in 2003, and he is being closely watched for how he might vote on the California ban. He cautioned in the Lawrence case that it had nothing to do with gay marriage, but dissenting Justice Scalia predicted the decision would lead to the invalidation of state laws against same-sex marriage.

Kennedy’s decision is widely cited in the briefs in support of same-sex unions.

The California couples, Kris Perry and Sandy Stier of Berkeley and Paul Katami and Jeff Zarrillo of Burbank, filed their federal lawsuit in May 2009 to overturn the same-sex marriage ban that voters approved the previous November. The ballot measure halted same-sex unions in California, which began in June 2008 after a ruling from the California Supreme Court.

Roughly 18,000 couples were wed in the nearly five months that same-sex marriage was legal and those marriages remain valid in California.

The case is Hollingsworth v. Perry, 12-144.