A significant blow came to the Defense of Marriage Act on Thursday when for the first time a federal appeals court ruled the anti-gay law was unconstitutional.

In a unanimous 3-0 decision, the U.S. First Circuit Court of Appeals in Massachusetts ruled that Section 3 of DOMA, which prohibit federal recognition of same-sex marriage, violated married same-sex couples rights under the U.S. Constitution.

“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,” the decision states. “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.”

The ruling was written U.S. Circuit Court Judge Michael Boudin, who heard the case along with Chief Circuit Judge Sandra Lynch and Circuit Judge Juan Torruella.

Lynch was appointed by a Democrat, former President Bill Clinton, but the other judges were appointed by Republicans. Torruella was appointed by former President Ronald Reagan and Boudin was appointed by former President George H.W. Bush.

The decision was brought down in two cases challenging DOMA: Gill v. Office of Personnel Management, which was filed by Gay & Lesbian Advocates & Defenders; and Commonwealth of Massachusetts v. Department of Health & Human Services, which was filed by Massachusetts Attorney General Martha Coakley.

The Gill lawsuit contends that DOMA is unconstitutional because it violates equal protection under the U.S. Constitution, while Commonwealth lawsuit argues DOMA violates a state’s right to regulate marriage under the Tenth Amendment.

In a statement, Coakley praised the ruling as evidence that DOMA violates married gay couples’ rights under the U.S. Constitution.

“Today’s landmark ruling makes clear once again that DOMA is a discriminatory law for which there is no justification,” Coakley said. “It is unconstitutional for the federal government to create a system of first- and second-class marriages, and it does harm to families in Massachusetts every day. All Massachusetts couples should be afforded the same rights and protections under the law, and we hope that this decision will be the final step toward ensuring that equality for all.”

Retiring gay Rep. Barney Frank, a Democrat who represents Massachusetts in Congress, also praised the ruling and expressed confidence the ruling against DOMA will be upheld when the case reaches the Supreme Court.

“The current situation, in which the rights of some couples married in other states are recognized, while the rights of other couples married in those states are denied, is clearly a violation of the U.S. Constitution,” Frank said. “I am confident that the U.S. Supreme Court will add its support for this decision which is so firmly grounded in long-standing American constitutional principles.”

The appeals court ruling affirms U.S. District Court Judge Joseph Tauro’s decision against DOMA in the two decisions he delivered in response to the lawsuits in July 2010. Since that time, President Obama has discontinued defending DOMA in court. Two other federal courts have also concluded that DOMA is unconstitutional.

Douglas NeJaime, who’s gay and a law professor at Loyola Law School, called the First Circuit decision “concise and relatively straightforward” and said the bigger question is whether this case or the challenge against California’s Proposition 8, Perry v. Brown, will first reach the Supreme Court.

“Whichever case makes it there first will have significant consequences for the same-sex marriage movement,” NeJaime said. “If Gill gives the Court its first bite at the same-sex marriage apple, the justices will address the more limited and potentially less controversial question of the federal rights of same-sex couples who are already married. Whereas Perry could present the Justices with broader questions of the rights of same-sex couples to marry, potentially across the nation.”

The Bipartisan Legal Advisory Group, under the direction of House Speaker John Boehner, voted on a party-line basis to take up defense of DOMA in the administration’s stead. It’s likely to appeal the decision either to the full First Circuit or the U.S. Supreme Court.

Oral arguments before the First Circuit took place April 4. Mary Bonauto, GLAD’s civil rights project director, represented her organization during the hearing. Massachusetts Assistant Attorney General Maura Healey argued on behalf of Massachusetts.

Defending DOMA in court was Paul Clement, a former U.S. solicitor general. After the Obama administration declared it would no longer defend DOMA, House Speaker John Boehner hired Clement to advocate for DOMA on behalf of the Bipartisan Legal Advisory Group, which voted along party lines to take up defense of the law.

Stuart Delery, who’s gay and the Justice Department’s acting assistant attorney general for the civil division, also litigated against the anti-gay law in court.

NOTE: This post has been edited and updated.