A key part of the law banning federal recognition of same-sex marriage was struck down as unconstitutional by a U.S. appeals court Thursday.
The Defense of Marriage Act -- known as DOMA -- defines marriage for federal purposes as unions exclusively between a man and woman.
At issue is whether the federal government can deny tax, health and pension benefits to same-sex couples in states where they can legally marry. The ruling is a boost for gay rights advocates and the Obama administration, which in a rare move, has refused to defend a federal law in court.
"If we are right in thinking that disparate impact on minority interests and federalism concerns both require somewhat more in this case than almost automatic deference to Congress' will, this statute fails that test," said the three judge panel.