A federal appeals court in Boston ruled against the Defense of Marriage Act Thursday, taking a significant step toward the highly anticipated Supreme Court challenge against the 1996 law that prohibits the federal government from recognizing legally married same-sex couples.

The unanimous decision by the three-judge panel of the First Circuit U.S. Court of Appeals marked the first time a federal appeals court has ruled the law unconstitutional. The court concurred with a lower court judge, U.S. District Judge Joseph Tauro, who ruled in 2010 that DOMA was unconstitutional because it denied same-sex married couples the benefits provided to straight married couples, such as the ability to file taxes jointly.

Jonathan Knight and Marlin Nabors, two of the 17 plaintiffs being represented by Gay and Lesbian Advocates and Defenders, which brought the DOMA challenge, said they were "thrilled" with the appeals court's decision. They have been married for five years.

"We're thrilled to learn that the law is on the same page now and we fall into the same class as everyone else," Knight said on a press call with reporters. "We're no longer second-class citizens."

Nabors said it was "validating that the appeals court judges sat around a conference table and came to the same conclusion that Jonathan and I have come to at our coffee table: that we deserve to be treated like every other couple."

Judge Michael Boudin, an appointee of President George H.W. Bush, wrote the opinion for the appeals court. The court stayed its decision in anticipation that "certiorari will be sought and that Supreme Court review of DOMA is highly likely."

"To conclude, many Americans believe that marriage is the union of a man and a woman, and most Americans live in states where that is the law today," wrote Boudin. "One virtue of federalism is that it permits this diversity of governance based on local choice, but this applies as well to the states that have chosen to legalize same-sex marriage. Under current Supreme Court authority, Congress' denial of federal benefits to same-sex couples lawfully married in Massachusetts has not been adequately supported by any permissible federal interest."

Mary Bonauto, the lead attorney for GLAD, said the decision recognized that the law passed by Congress represents an "unprecedented federal intrusion and federal intervention" that "burdens" states and "targets" same-sex couples, a vulnerable group.

"We think this is a fantastic decision that is crisp and solid and well reasoned," she said. "It's really rooted in the last 50 years of equal protection jurisprudence and federalist jurisprudence. We think the decision is well anchored, and this decision points out some of the unique vulnerabilities of DOMA."

Oral arguments were heard last month in the consolidated appeal of two cases, Gill v. Office of Personnel Management and Commonwealth of Massachusetts v. Department of Health and Human Services. The cases challenge Section 3 of DOMA, which holds that "the word 'marriage' means only the legal union of a man and a woman as husband and wife, and the word 'spouse' refers only to a person of the opposite sex who is a husband or a wife."

During the arguments, GLAD argued that DOMA violates equal protection. The state of Massachusetts anchored its case in Tenth Amendment violations. Paul Clement, the former U.S. solicitor general arguing on behalf of the House Republicans' Bipartisan Legal Advisory Group, defended DOMA as having a "rational basis" at the time of its passage. BLAG took over the law's defense after the Obama administration announced the Department of Justice would stop defending DOMA last year.

The appeals court applied intensified rational basis review in its analysis, meaning that the federal government's interest in maintaining DOMA must be shown with special clarity. The court found those reasons lacking. DOMA does not pertain to child-rearing, for example, as Clement argued for BLAG on a "rational basis," and the court rejected the idea that DOMA somehow reinforces heterosexual marriages.

The DOMA case differs from another high-profile legal challenge, the case against Proposition 8 in California, which is still pending review in the Ninth Circuit Court of Appeals. That case concerns whether a fundamental right to same-sex marriage exists. GLAD's team members said they did not expect that issue to be considered should their challenge reach the high court.

"What we're dealing with in this case is the question of how the federal government treats people who are already married in their own states," said Bonauto. "It's the marriages of same-sex couples alone that have been singled out for sweeping disrespect by the federal government. What we really have here is a classic double standard."

The decision concludes activity in the six-member appeals court, according to Bonauto. She said that her team fully expected the Supreme Court would hear the case, noting that "every expert in the country" believes in that prospect.

Petitions for certiorari are due within 90 days, after which point GLAD will have 30 days to reply. That timeline means the Supreme Court could be considering in October whether to hear the case. The high court will not be bound by the framing of the First Circuit, according to Bonauto.

"All issues are raised to the court, including intermediate scrutiny or heightened scrutiny," she said. "We think this is a very solid decision to go before the Supreme Court."

Marriage equality advocates hailed the decision in the First Circuit.

"Today's unanimous decision issued by the First Circuit Court of Appeals is a powerful affirmation that the so-called Defense of Marriage Act is an unconstitutional and unjust law whose days are numbered," said Evan Wolfson, founder and president of Freedom to Marry. "This ruling will return the federal government to its historic role of respecting marriages performed in the states, without carving out a 'gay exception' that denies thousands of protections."

Sen. Kirsten Gillibrand of New York, a cosponsor of legislation to repeal DOMA, called for bipartisan action in Congress. The Senate Judiciary Committee approved the Respect for Marriage Act last year, but the Republican-led House has not considered the measure.

"If Democrats and Republicans can come together to do what's right in New York, I know we can do the same in Congress to do what's right for all of America," she said. "Now is the time to act on the federal level by repealing this discriminatory and unconstitutional law."