WASHINGTON—The U.S. Supreme Court effectively expanded the right of gay marriage to more than half the nation Monday as it let stand lower-court rulings that struck down bans in five states.

The court’s surprise move, declining to consider the states’ requests to reinstate the prohibitions, signaled the justices feel no urgency to rule on whether same-sex couples nationwide have a constitutional right to marry.

The justices turned away appeals from Indiana, Oklahoma, Utah, Virginia and Wisconsin, all of which saw their bans on same-sex marriage thrown out by three different U.S. appeals courts in recent months.

While the justices neither approved nor rejected the appeals court rulings against gay-marriage bans, “sometimes they act by inaction,” said Theodore Olson, an attorney who represents same-sex marriage proponents in the Virginia case.The Supreme Court, as is its custom, offered no explanation for why it declined to intervene.

The five states responded quickly, with county clerks starting to issue same-sex marriage licenses within hours.

Although Oklahoma is complying, Gov. Mary Fallin, a Republican, criticized the high court, saying, “The will of the people has now been overridden by unelected federal justices, accountable to no one.”

Utah Gov. Gary Herbert, a fellow Republican, said that “I believe states should have the right to determine their own laws regarding marriage. That said, we are a society of laws and we will uphold the law.”

Virginia Gov. Terry McAuliffe, a Democrat who backs gay marriage, said his state “is already well-prepared to implement this historic decision.”

The Supreme Court’s action surprised both sides, especially because antigay-marriage states and gay-rights advocates alike had urged the court to step in. Observers had expected a same-sex marriage case to be the marquee decision of the court’s 2014-15 term, which began Monday.

It takes only four votes to accept a case for argument, so Monday’s orders, published with no dissents, suggested tactical moves by both sides.

Justice Anthony Kennedy and the court’s liberals, whose opinions have leaned toward same-sex marriage, may have withheld delivering a final blow for fear of outpacing public opinion that already is resolving the controversy in favor of gay rights.

The court’s conservative wing may be placing bets that the Cincinnati-based Sixth U.S. Circuit Court of Appeals will uphold state restrictions, providing a stronger engine for an uphill climb against the expansion of marriage rights.

But Kermit Roosevelt, a law professor at the University of Pennsylvania, said the court’s move may suggest the marriage issue is all but decided. “There is no way the Supreme Court would allow those decisions to stand and go into effect if it was eventually going to rule the other way,” Mr. Roosevelt said.

Opponents of same-sex marriage voiced dismay. “I think it really borders on an abdication of the court’s responsibility for constitutional adjudication,” said lawyer Charles Cooper, who previously represented supporters of a California voter proposition banning same-sex marriage. Last year, the Supreme Court let stand a lower-court ruling voiding that measure.

Among the earlier decisions, the 10th U.S. Circuit Court of Appeals in Denver invalidated bans in Utah and Oklahoma after finding same-sex couples “are entitled to exercise the same fundamental right as is recognized for persons who wish to marry a person of the opposite sex.”

Colorado, also in the 10th Circuit, said it would drop its defense of its marriage restriction and quickly expand matrimony to same-sex couples. In neighboring Kansas, the attorney general said his office was closely watching the legal developments. Wyoming’s attorney general didn’t immediately respond to a request for comment.

Virginia’s ban was struck down by the Fourth U.S. Circuit Court of Appeals, which also covers North Carolina, South Carolina and West Virginia. South Carolina’s attorney general vowed to continue defending its same-sex marriage ban, while the North Carolina and West Virginia attorneys general said they were reviewing their options.

Meanwhile, gay-rights advocates began filing court papers seeking to force all states in the covered circuits to expand marriage rights immediately.

Evan Wolfson, president of the group Freedom to Marry, said the Supreme Court’s action meant 60% of the American public lives in states where gay marriage could be legal. “But we are one country, with one Constitution, and the court’s delay in affirming the freedom to marry nationwide prolongs the patchwork of state-to-state discrimination,” he said.

No appeals court has upheld a gay-marriage ban since a 2013 Supreme Court ruling called such restrictions into question, but other cases are in the pipeline, including the Sixth Circuit litigation, which covers laws in Kentucky, Michigan, Ohio and Tennessee.

If a future appeals court ruling upholds a state same-sex marriage ban, creating a split in the circuit courts, it is likely the Supreme Court would have to resolve the disagreement

Mr. Olson said Monday’s orders could influence lower court judges considering similar cases.

“If I were a federal judge in one of those other jurisdictions, I think I could read the handwriting on the wall,” said Mr. Olson, a U.S. solicitor general under President George W. Bush.

Decisions invalidating gay-marriage bans have relied on the Supreme Court’s June 2013 ruling in U.S. v. Windsor, which struck down the 1996 federal Defense of Marriage Act’s denial of federal benefits for same-sex married couples. That 5-4 ruling split the court along ideological lines, with Justice Kennedy, a moderate conservative, joining the court’s liberal bloc to invalidate the law.

The Windsor majority said Congress had no legitimate purpose for undermining same-sex marriage in states allowing it. The justices on the same day, in a case on the California voter initiative, sidestepped whether the Constitution allows states to continue to ban such marriages.

Justice Ruth Bader Ginsburg hinted at Monday’s outcome in September remarks at the University of Minnesota Law School. Because appeals courts so far have been unanimous in striking down same-sex marriage bans, there was “no need for us to rush” back into the issue, she said. But she added, “Sooner or later, yes, the question will come to the court.

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