In a strongly worded legal brief, the Obama administration has said the federal act that defines marriage as being between a man and a woman was motivated by hostility toward gays and lesbians and is unconstitutional.

The brief was filed Friday in federal court in San Francisco in support of a lesbian federal employee's lawsuit claiming the government wrongly denied health insurance coverage to her same-sex spouse.

The Justice Department says Karen Golinski's suit should not be dismissed because the law under which her spouse was denied benefits — the Defense of Marriage Act — violates the U.S. Constitution's guarantee of equal protection.

"The official legislative record makes plain that DOMA Section 3 was motivated in large part by animus toward gay and lesbian individuals and their intimate relationships, and Congress identified no other interest that is materially advanced by Section 3," the brief reads, referring to the section in the act that defines marriage as being between a man and a woman.

Though the administration has previously said it will not defend the marriage act, the brief is the first court filing in which it urges the court to find the law unconstitutional, said Tobias Barrington Wolff, a law professor at the University of Pennsylvania.