(CNN) -- A legal advocacy group sued the federal government Tuesday, seeking benefits for 15 gay and lesbian Massachusetts residents who wed after the state legalized same-sex marriage.

A lawsuit contends the federal government is denying benefits to Massachusetts same-sex couples.

Gay & Lesbian Advocates & Defenders -- the same Boston-based group that successfully argued in 2003 for same-sex marriage rights in Massachusetts -- filed the lawsuit Tuesday in U.S. District Court in Boston on behalf of six couples and three men whose husbands have died.

The suit contends the federal Defense of Marriage Act denies them protections and benefits that heterosexual couples receive, including health insurance for federal employees, the ability to file as "married filing jointly" on federal income taxes and Social Security spousal protections.

Two other couples plan to file suit after they receive an anticipated rejection of their amended tax returns from the Internal Revenue Service, GLAD said.

President Clinton signed the Defense of Marriage Act into law in 1996. It effectively bars the federal government from recognizing same-sex unions by defining marriage as "a legal union between one man and one woman as husband and wife" and a spouse as "a person of the opposite sex who is a husband or a wife."

GLAD's suit addresses only Section 3 of the law, a portion that prevents the federal government from giving Social Security and other protections to same-sex married couples. GLAD says there are 1,138 federal laws in which marital status is a factor.

Only Massachusetts and Connecticut allow same-sex marriage. Vermont, New Jersey and New Hampshire allow civil unions.

One of the plaintiffs in the suit is Dean Hara, who was married to former U.S. Rep. Gerry E. Studds, the first openly gay member of Congress. The two legally married in 2004, a week after Massachusetts began issuing marriage licenses to same-sex couples. Studds, who by then had retired from politics, died in October 2006.

"Gerry was a public servant for 27 years, worked hard for our country and paid as much into the system as anyone else," Hara, 51, said, according to the GLAD Web site.

"But after he died, I was treated differently than other surviving spouses. Every federal employee counts on their surviving spouses having basic protections, but the federal government denies me those protections because of DOMA."

Other plaintiffs include state trooper Sgt. Mary Ritchie and her spouse, Kathy Bush. They said not being able to file federal taxes jointly denies them deductions afforded to heterosexual married couples.

"We work hard, pay taxes, volunteer and do our part for our community. But the federal government still tells us we're less of a family than other families in our neighborhood -- families Mary works to protect," Bush said, according to the GLAD Web site.

Others named in the suit include a postal worker who hasn't been allowed to add her spouse to her health plan, a widower who hasn't been able to collect his spouse's Social Security payments and a Social Security Administration retiree who is denied health insurance for his spouse.

The United States of America and several federal agencies are named as defendants.

All About Same-Sex Marriage • Massachusetts