Correction: An earlier version of this article misspelled the last name of Kate Kendell, executive director of the National Center for Lesbian Rights.

The nationwide legal battle over same-sex marriage escalates Thursday when a federal appeals court reviews the first in a string of unanimous judicial rulings that state bans on gay marriage cannot stand in the wake of last summer’s Supreme Court action.

A three-judge panel of the U.S. Court of Appeals for the 10th Circuit in Denver will be considering Utah’s ban on same-sex marriage, which was struck down in December by a federal district judge in Salt Lake City.

It is a review soon to be repeated around the country, the intermediate step in returning a question to the Supreme Court that the justices avoided the first time around — whether marriage is a fundamental right that under the Constitution may not be denied to same-sex couples.

Both the speed with which the legal challenges are proceeding — pending appeals court action in this case and in Virginia could tee up the issue in time for the Supreme Court term that begins in October — and the change in public approval of same-sex marriage have taken even gay rights groups by surprise.

Kate Kendell, executive director of the National Center for Lesbian Rights, pronounced herself “really awestruck” at the change.

“It goes without saying we are in a quite amazing moment,” she said.

Proponents of the traditional definition of marriage as between a man and woman are looking to the coming appeals court reviews to blunt what Evan Wolfson, founder of the organization Freedom to Marry, calls the “irrefutable momentum” that the district court rulings have provided.

Ryan T. Anderson, a fellow at the Heritage Foundation, said he hopes the appeals courts will recognize that states have played the leading role in defining marriage and domestic relations and that such decisions should be left to the people and their representatives.

“You can be for same-sex marriage, you can be against same-sex marriage,” Anderson said. “But the Constitution doesn’t settle that question.”

U.S. District Judge Robert Shelby in Salt Lake City disagreed. In December, he became the first judge to strike down a state ban after the Supreme Court’s 5 to 4 decision in June that found part of the federal Defense of Marriage Act unconstitutional.

The majority said the federal government cannot not deny federal benefits and recognition to same-sex couples who were married in states where gay nuptials are legal. The court said Congress’s decision to define marriage as only between a man and a woman had no rationale other than a discriminatory moral disapproval of gay couples.

Shelby said the reasoning behind the court’s decision compelled him to strike down Utah’s ban on gay marriage, which 66 percent of those voting added to the state constitution in 2004.

“The State’s current laws deny its gay and lesbian citizens their fundamental right to marry and, in so doing, demean the dignity of these same-sex couples for no rational reason,” Shelby wrote.

Since then, judges in Virginia, Oklahoma, Texas and Michigan have made similar findings. Courts in Tennessee, Ohio and Kentucky have said those states must recognize gay marriages performed elsewhere.

The Utah decision was a “game-changer,” in the words of Peggy Tomsic, the attorney representing the three couples who challenged the ban, not only because it was first and in a deeply conservative state but because of what happened next.

Shelby and then a panel of the 10th Circuit turned down Utah’s request to put the decision on hold while it was appealed. More than 1,200 couples were wed before the Supreme Court approved Utah’s stay request 17 days later.

The justices gave no reason for granting the stay and thus shed no more light on the court’s June decision in U.S. v. Windsor that has prompted the legal action.

That ruling provides arguments for both sides.

On one hand, the opinion written by Justice Anthony M. Kennedy and joined by the court’s four liberals noted that defining marriage is traditionally a power reserved for the states.

On the other, the opinion dismissed Congress’s arguments as to why the federal government should recognize only traditional definitions of marriage. It said the arguments were mostly window dressing for unlawful prejudice based on sexual orientation.

To defend its law at the appeals court, Utah hired Gene C. Schaerr, an experienced Washington appellate lawyer, who resigned from the law firm Winston & Strawn to take on what he seemed to describe as a higher mission.

“I have accepted that position so that I can fulfill what I have come to see as a religious and family duty: defending the constitutionality of traditional marriage in the state where my church is headquartered and where most of my family resides,” Schaerr, who is Mormon, wrote in an e-mail reported by the Web site Above the Law.

In the state’s briefs, Schaerr writes that Shelby misread the key holding of the Windsor decision, which is that states hold the key to defining marriage.

Utah’s choice of a “man-woman” model for marriage “is not intended to demean other family structures, any more than giving an ‘A’ to some students demeans others,” according to the briefs.

He said the state’s rationale for the decision is that Utah’s interest in marriage is providing the best environment for children, rather than “the novel principle that marriage is whatever emotional bond any two (or more) people say it is.”

In enacting the constitutional amendment, according to the briefs, Utah voters were expressing their “firm belief — also supported by sound social science — that moms and dads are different, not interchangeable, and that the diversity of having both a mom and a dad is the ideal parenting environment.”

And the briefs noted that Utah’s history, which included polygamy, gives it special reason for its definition of marriage.

“To become a State, Utah had to adopt a virtually ‘irrevocable’ State constitutional provision that ‘forever prohibited’ polygamous marriage and thereby made adherence to monogamous marriage (at the time undoubtedly understood as between one man and one woman) the only alternative,” according to the briefs.

Austin R. Nimocks, senior counsel at the Alliance Defending Freedom, a conservative legal organization involved in many of the gay-marriage cases, said he is hopeful that the appeals courts will take note of the history of state definitions of marriages, not just the relatively recent moves to enshrine them in state constitutions.

The amendments “did not change the substance of the laws,” which have been around in some cases for centuries, he said.

Tomsic and her clients’ supporters — there are dozens of friends-of-the-court briefs filed on both sides of the case — say opponents of gay marriage have offered no new rationales.

The procreation argument already has been dismissed by the Supreme Court’s majority, they said. And the “sound social science” that purports to show those raised in households headed by parents of the same sex do worse than others has been discredited, they say.

One study often cited by opponents of same-sex marriage was trashed by a federal judge in Michigan who held a trial on that state’s ban. He called testimony by the study’s author “entirely unbelievable and not worthy of serious consideration.”

Same-sex marriage is legal in 17 states and the District of Columbia. The rest of the states prohibit the marriages through state law or constitutional amendments.

There are lawsuits challenging those restrictions in all but five states — Alaska, Georgia, Montana, North Dakota and South Dakota. Appeals of judicial decisions regarding the bans are pending in five of the circuit courts of appeals into which the country is divided.

A randomly selected panel of three 10th Circuit judges will hear the Utah case and, a week later, the appeal regarding Oklahoma. The U.S. Court of Appeals for the 4th Circuit in Richmond is scheduled to hear arguments May 13 reviewing Norfolk-based U.S. District Judge Arenda L. Wright Allen’s decision striking down Virginia’s laws.

The veteran 10th Circuit panel consists of Judges Paul J. Kelly Jr., nominated by President George H.W. Bush; Carlos F. Lucero, chosen by President Bill Clinton; and Jerome A. Holmes, who was nominated by President George W. Bush.

The 10th Circuit’s ruling would be binding in the states it covers: Colorado, Kansas, New Mexico, Oklahoma, Utah and Wyoming.

But New Mexico will not be affected. Its highest court already took note of the Windsor decision and ruled unanimously that the state must allow gay marriages.