The Supreme Court on Monday decided to let stand rulings that allow same-sex marriage in Virginia, Utah, Oklahoma, Indiana and Wisconsin, a move that may dramatically expand across the nation a decades-long movement legalizing such unions.

The court’s action marks a turning point. A majority of Americans now live in states where gay couples can wed, and the court’s decision could soon bring the number of those states to 30, meaning there may be no going back.

“I’m blown away by this,” said James Esseks, a lawyer who heads the American Civil Liberties Union’s legal efforts concerning same-sex marriage. “It is a watershed moment for the entire country.”

The court’s decision came without explanation and startled those on both sides of the issue who had urged the justices to accept the cases to rule on the constitutionality of marriage in a way that affected all 50 states.

The denials of cases­ from three federal appeals courts carry no formal instruction for other lower courts still considering challenges to state bans; if one of them upholds such a measure, the issue could return to the high court.

Same-sex marriage status in the U.S., state-by-state

But gay rights supporters said the message from the court was that it found no reason to re­instate the voter-approved bans that had been struck down in a series of rulings built from the Supreme Court’s 2013 decision invalidating part of the federal Defense of Marriage Act.

The decision is likely to expand same-sex marriage to other states covered by the federal appeals courts that already have ruled that the bans are unconstitutional: Colorado, Wyoming, Kansas, West Virginia, North Carolina and South Carolina.

Esseks questioned whether the court would allow thousands of new same-sex marriages if there was a chance it could ultimately find states have the right to define marriage as only between a man and a woman.

But opponents of same-sex marriage said the fight is not over.

“The court’s decision not to take up this issue now means that the marriage battle will continue,” said Byron Babione, senior counsel at the Alliance Defending Freedom, which has been active in defending prohibitions against same-sex marriage. “The people should decide this issue, not the courts.”

Within hours of the court’s action, same-sex couples were married in Fairfax County, Va., headed to the courthouse in Salt Lake City and issued licenses in Pueblo, Colo. Those who live in the affected states but whose marriages were performed elsewhere suddenly found themselves officially hitched in their home towns.

Kathryn Hamm and Amy Walter live in Arlington but eloped to the District to get married in November. Walter was in London on business when the news broke Monday morning, so Hamm texted instead of calling.

“We are officially married!!!!!!!!!!” Hamm typed.

Virginia Attorney General Mark R. Herring (D), who refused to defend the state’s ban and joined challengers to overturn it, held a news conference in front of the Arlington County Courthouse to announce that same-sex couples can begin marrying.

“This is the outcome that we have hoped for, it is the outcome that we have fought for and it is the outcome that the Constitution requires,” he said. “Today’s decision will change the lives of thousands of loving couples, their children and their families in a positive and transformative way.”

The decision by the justices not to take the cases stunned those who closely watch the court. Even though no appeals court had upheld a state ban — and such disagreements between federal circuit courts usually are precursors to Supreme Court review — the court had been asked by more than 30 states to take up the issue.

Moreover, the justices had put on hold decisions allowing marriages in Virginia and Utah, which was seen as a sign the court did not want to allow more marriages until it had reviewed the issue.

Ed Whelan, an influential commentator in the conservative legal establishment, wrote that “the court’s denial of review in all the pending cases strikes me as grossly irresponsible, as a huge abdication of duty on the part of at least six justices.” At least four of the nine justices must agree in order to take up a case, although five are needed to win.

Some liberal groups also thought the court had shirked its role.

Alliance for Justice President Nan Aron said in a statement that in gay rights victories in 2013, “the Supreme Court began to bend the arc of history toward justice on this issue. By declining to take these cases, the court passed up an opportunity to finish the job.”

There are challenges to same-sex marriage prohibitions in every state. Two circuit courts, in Cincinnati and San Francisco, have heard appeals from states whose bans were struck and could rule at any time.

But Esseks said that Monday’s action indicates that it does not matter whether an appeals court rules that state prohibitions are constitutional.

“If that happened, the court will clearly take the case and decide the issue,” Esseks said. “But [Monday’s decision] is more than a hint about what the court will do.”

In June 2013, the court struck down part of the Defense of Marriage Act, which had denied federal recognition of same-sex marriages performed in states where it was legal. Although that decision did not touch on whether state bans were unconstitutional, a number of federal court decisions have ruled since then that the reasoning of the opinion written by Justice Anthony M. Kennedy commands such a finding.

The split in federal and state courts since last year’s ruling in U.S. v. Windsor has been 40 to 2 that state prohibitions violate the Constitution’s guarantees of due process and equal protection.

The Windsor decision was decided on a 5 to 4 vote, with the court’s liberals — Justices Ruth Bader Ginsburg, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan — joining Kennedy.

Since it takes only four votes to grant review of lower-court decisions, that means at least one of those who voted no in Windsor — Chief Justice John G. Roberts Jr. and Justices Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr. — were unwilling to force the court to take up the issue now.

In his dissent in that case, Roberts made clear that the court was not passing judgment on whether the Constitution’s guarantee of due process and equal protection meant that state marriage bans were unconstitutional.

But Scalia wrote his own blistering objection that said the majority had laid the groundwork for potential challengers.

“It takes real cheek for today’s majority to assure us, as it is going out the door, that a constitutional requirement to give formal recognition to same-sex marriage is not at issue here,” Scalia wrote.

Instead, “the majority arms well every challenger to a state law restricting marriage to its traditional definition,” Scalia wrote, and such suits are a “second . . . shoe to be dropped later.”

Justin Jouvenal, Jenna Portnoy, Reid Wilson and Niraj Chokshi contributed to this report.