(Boston) Massachusetts, the first state to legalize gay marriage, sued the U.S. government Wednesday over a federal law that defines marriage as a union between a man and a woman.

The federal Defense of Marriage Act interferes with the right of Massachusetts to define and regulate marriage as it sees fit, Massachusetts Attorney General Martha Coakley said. The 1996 law denies federal recognition of gay marriage and gives states the right to refuse to recognize same-sex marriages performed in other states.

The lawsuit, filed in federal court in Boston, argues the act “constitutes an overreaching and discriminatory federal law.” It says the approximately 16,000 same-sex couples who have married in Massachusetts since the state began performing gay marriages in 2004 are being unfairly denied federal benefits given to heterosexual couples.

Besides Massachusetts, five other states – Connecticut, Vermont, New Hampshire, Maine and Iowa – have legalized gay marriage. Gay marriage opponents in Maine said Wednesday that they had collected enough signatures to put the state’s new law on the November ballot for a possible override.

The Massachusetts lawsuit challenges the section of the federal law that creates a federal definition of marriage as “a legal union between one man and one woman as husband and wife.”

Before the law was passed, Coakley said, the federal government recognized that defining marital status was the “exclusive prerogative of the states.” Now, because of the U.S. law’s definition of marriage, same-sex couples are denied access to benefits given to heterosexual married couples, including federal income tax credits, employment benefits, retirement benefits, health insurance coverage and Social Security payments, the lawsuit says.

The lawsuit also argues that the federal law requires the state to violate the constitutional rights of its citizens by treating married heterosexual couples and married same-sex couples differently when determining eligibility for Medicaid benefits and when determining whether the spouse of a veteran can be buried in a Massachusetts veterans’ cemetery.

“In enacting DOMA, Congress overstepped its authority, undermined states’ efforts to recognize marriages between same-sex couples, and codified an animus towards gay and lesbian people,” the lawsuit states.

The Justice Department had not seen the lawsuit and cannot respond until it has a chance to review it, spokesman Charles Miller said.

The Defense of Marriage Act was enacted when it appeared Hawaii would soon legalize same-sex marriages and opponents worried that other states would be forced to recognize them.

President Barack Obama has pledged to work to repeal the law, although gay rights activists criticized the administration last month after Justice Department lawyers defended it in a court brief. White House aides said they were doing their jobs to support a law that is on the books.

This is the second lawsuit filed in Massachusetts challenging the law.

In March, the Boston-based Gay & Lesbian Advocates & Defenders claimed the law discriminates against gay couples and is unconstitutional because it denies them access to federal benefits that other married couples receive, such as health insurance and pensions.

In Maine, the Stand for Marriage Maine coalition said it took only four weeks to gather more than the 55,087 signatures necessary to put gay marriage to a vote.

The Maine law to legalize gay marriage had been scheduled to go into effect Sept. 12. It will be put on hold after the signatures are submitted and certified by the secretary of state’s office. Voters will then decide in November whether it should stand.