The first state to legalize gay marriage became the first to challenge the U.S. Defense of Marriage Act, contending that the 1996 law interferes with Massachusetts' right to define and regulate marriage.
The law defines marriage as a union between a man and a woman. It 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, says that the approximately 16,000 same-sex couples who have wed in Massachusetts since the state began performing gay marriages in 2004 are being unfairly denied federal benefits given to heterosexual couples.
"They are entitled to equal treatment under the laws regardless of whether they are gay or straight," state Atty. Gen. Martha Coakley said at a news conference.
Connecticut, Vermont, New Hampshire, Maine and Iowa have legalized gay marriage. California did too, but Proposition 8 reversed it.