Although not unanimous, the 10th Circuit ruling marks a major victory for equal-rights advocates, as it is the first federal appeals court to declare a constitutional right to marry regardless of sex.
The decision applies to all six states covered by the 10th Circuit: Colorado, Kansas, New Mexico, Oklahoma, Utah, and Wyoming.
"This is a whole new playing field now. It's hugely momentous for Utah and the entire country," said Shannon Minter, one of the attorneys in the case.
The justices immediately put their ruling on hold pending appeal, and Utah's attorney general confirmed that he would keep the case moving.
Justice Paul J. Kelly Jr. wrote in a dissenting opinion that the court was overstepping its authority and that states should be able to decide who can marry.
"We should resist the temptation to become philosopher kings, imposing our views under the guise of the Fourteenth Amendment," Kelly wrote.
A similar warning was sounded by Justice Antonin Scalia last summer, when the U.S. Supreme Court overturned the federal Defense of Marriage Act.
Since that decision - exactly one year ago - 16 federal judges have issued rulings siding with gay-marriage advocates. At least a dozen cases are pending before state and federal courts, and experts say the wave of litigation will not subside until the Supreme Court settles the issue. There have been no federal rulings against same-sex marriage.
The Utah Attorney General's Office said in a statement that although the court did not rule in its favor, "we are pleased that the ruling has been issued, and takes us one step closer to reaching certainty and finality . . . with a decision from the highest court."
The state's Republican governor, Gary Herbert, agreed. "We can't get finality," he said, "and final resolution unless the Supreme Court hears the case, and makes a decision."
The Supreme Court, which is wrapping up its 2014 session, has not indicated whether it will take up the issue next year. If it does, it may have several decisions from which to choose.
The U.S. Court of Appeals for the Fourth Circuit heard arguments from a Virginia case in May and is expected to issue a ruling soon. The Sixth Circuit appeals court has scheduled arguments Aug. 6 in a Kentucky case.
In Indiana on Wednesday, a U.S. District Judge struck down that state's ban in a ruling that immediately allowed gay couples to wed. The state plans to appeal.
In Pennsylvania, U.S. District Judge John E. Jones III in May overturned a state law that defined marriage as between one man and one woman and refused to recognize same-sex marriages performed in other states.
Unlike his counterpart in Utah, Gov. Corbett declined to appeal the case. A Schuylkill County row officer, Theresa Santai-Gaffney, is now asking the Third Circuit appeals court to allow her to defend the state's Marriage Law.
Santai-Gaffney has also requested that that court follow the 10th Circuit Court and put same-sex marriage on hold until appeals are complete.
In filings Thursday, the Corbett administration said it was taking no position on Santai-Gaffney's requests, and the plaintiffs argued that as a county clerk, Santai-Gaffney lacks standing to defend a state law.
At least two other cases in Pennsylvania state courts - filed by same-sex couples who married in Montgomery County in 2013, before Jones' ruling legalized the practice - were suspended Friday until appeals are complete the federal case.
In Allegheny County, which includes Pittsburgh, county officials are not waiting for appeals. After Jones' ruling, county employees were notified that same-sex partners would no longer be eligible for county benefits, but spouses would.
County spokeswoman Amie Downs said the change comes "in order to ensure that all employees are being treated equally and fairly." She said the 11 affected employees can provide proof of marriage by July 31 to keep their spouses on the county plan.
This article contains information from the Associated Press.