BOSTON - An appeals court ruled Thursday that the heart of a law that denies a host of federal benefits to gay married couples is unconstitutional.

The 1st U.S. Circuit Court of Appeals in Boston said the Defense of Marriage Act, which defines marriage as a union between a man and a woman, discriminates against married same-sex couples by denying them federal benefits.

The law was passed in 1996 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 court didn't rule on the law's other provision, which said states without same-sex marriage cannot be forced to recognize gay unions performed in other states.

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. Massachusetts, Connecticut, New York, Iowa, New Hampshire, Vermont, Maryland, Washington state and the District of Columbia have approved it, but Maryland and Washington's laws aren't 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.