BOSTON (AP) — The Justice Department on Friday asked a federal judge in Boston to dismiss a lawsuit that claims a federal law defining marriage as a union between a man and a woman is unconstitutional because it denies gay couples access to federal benefits given to other married couples.

In court documents, the Justice Department makes it clear the Obama administration thinks the law is discriminatory and should be repealed. But the department, calling the law "constitutionally permissible," said it has an obligation to defend federal laws when they are challenged in court.

The 1996 federal Defense of Marriage Act, known as DOMA, bars federal recognition of gay unions and denies gay couples access to pensions, health insurance and other government benefits.

The law was passed by Congress at a time when it appeared Hawaii would become the first state to legalize same-sex marriage. Opponents worried that other states would be forced to recognize such marriages.

Since then, six states have enacted laws or issued court rulings that permit same-sex marriage, including Massachusetts, Maine, Vermont, Connecticut and Iowa. New Hampshire's law takes effect Jan. 1, 2010.

The Massachusetts lawsuit was brought by seven gay couples and three widowers, all of whom were married in Massachusetts after it became the first state in the country to legalize gay marriage in 2004. They argue that DOMA violates the equal-protection clause of the U.S. Constitution because it treats married gay couples differently than other married couples.

Beatrice Hernandez and Melba Abreu, plaintiffs in the lawsuit, have been married for five years, but they aren't allowed to file a joint tax return, as heterosexual married couples can. Hernandez said they paid nearly $20,000 more in taxes between 2004 and 2007 than they would have if they had been able to file joint returns.

"It really is separate and unequal treatment," Hernandez said. "When we were able to marry in 2004, we didn't receive a different marriage certificate. We received one that was equal for all citizens here in Massachusetts."

The Justice Department, however, argues that there is no fundamental right to marriage-based federal benefits and says Congress is entitled to address issues of social reform on an "incremental" basis.

"Congress is therefore permitted to provide benefits only to those who have historically been permitted to marry, without extending the same benefit to those only recently permitted to do so," the government argued in its written response to the lawsuit.

"Congress may subsequently decide to extend federal benefits to same-sex marriages, and this Administration believes that Congress should do so. But its decision not to do so to this point is not irrational or unconstitutional."

Gay & Lesbian Advocates & Defenders, the legal group that filed the lawsuit, said DOMA is an exception to a long history of the federal government deferring to determinations by the states as to what constitutes marriage.

"We're seeking justice for the widows and widowers who are denied death benefits, for people who can't get on their spouse's health plan, for parents who can't file taxes jointly and pay thousands extra each year that they could put away for their children's education or family emergencies," said Gary Buseck, GLAD's legal director.

A bill to repeal DOMA was introduced in the U.S. House of Representatives on Tuesday by U.S. Rep. Jerrold Nadler, D-N.Y., but has little chance of making it to a vote this year.