In its unanimous ruling, the three-judge panel of the 1st U.S. Circuit Court of Appeals in Boston said the 1996 law that defines marriage as a union between a man and a woman discriminates against gay couples because it doesn’t give them the same rights and privileges as heterosexual couples.

The court didn’t rule on the law’s other politically combustible provision, which said states without same-sex marriage cannot be forced to recognize gay unions performed in states where it’s legal. It also wasn’t asked to address whether gay couples have a constitutional right to marry.

The law was passed at a time when it appeared Hawaii would legalize gay marriage. Since then, many states have instituted their own bans on gay marriage, while eight states have approved it, led by Massachusetts in 2004.

The appeals court agreed with a lower court judge who ruled in 2010 that the law is unconstitutional because it interferes with the right of a state to define marriage and denies married gay couples federal benefits given to heterosexual married couples, including the ability to file joint tax returns.

The 1st Circuit said its ruling wouldn’t be enforced until the U.S. Supreme Court decides the case, meaning that same-sex married couples will not be eligible to receive the economic benefits denied by DOMA until the high court rules.

That’s because the ruling only applies to states within the circuit, including Massachusetts, Rhode Island, Maine, New Hampshire and Puerto Rico. Only the Supreme Court has the final say in deciding whether a law passed by Congress is unconstitutional.

During arguments before the court last month, a lawyer for gay married couples said the law amounts to “across-the-board disrespect.” The couples argued that the power to define and regulate marriage had been left to the states for more than 200 years before Congress passed DOMA.

An attorney defending the law argued that Congress had a rational basis for passing it in 1996, when opponents worried that states would be forced to recognize gay marriages performed elsewhere. The group said Congress wanted to preserve a traditional and uniform definition of marriage and has the power to define terms used to federal statutes to distribute federal benefits.

Since DOMA was passed in 1996, many states have instituted their own bans on gay marriage, while eight states have approved it, including Massachusetts, Connecticut, New York, Iowa, New Hampshire, Vermont, Maryland, Washington state and the District of Columbia. Maryland and Washington’s laws are not yet in effect and may be subject to referendums.

Last year, President Barack Obama announced the U.S. Department of Justice would no longer defend the constitutionality of the law. After that, House Speaker John Boehner convened the Bipartisan Legal Advisory Group to defend it.

Gay & Lesbian Advocates & Defenders, the Boston-based legal group that brought one of the lawsuits on behalf of gay married couples, said the court agreed with the couples that it is unconstitutional because it takes one group of legally married people and treats them as “a different class” by making them ineligible for benefits given to other married couples.

“We’ve been working on this issue for so many years, and for the court to acknowledge that yes, same-sex couples are legally married, just as any other couple, is fantastic and extraordinary,” said Lee Swislow, GLAD’s executive director.