A federal appeals court in Boston today declared unconstitutional the federal Defense of Marriage Act, which defines marriage as a union of only a man and a woman.

“That’s the promise of America. We won,” said Mary Bonauto, attorney for Gay & Lesbian Advocates & Defenders, the Boston-based advocacy group that had challenged the 1996 law.

The federal law, known as DOMA, defines marriage as a union of a man and a woman when it comes to federal laws and regulations, consequently limiting federal benefits such as Social Security survivor benefits and the ability to file joint taxes to heterosexual couples.

Advertisement - Continue Reading Below

Massachusetts Attorney General Martha Coakley lauded the ruling by the First US Circuit Court of Appeals, which came nine years after the state’s Supreme Judicial Court became the first court in the Western Hemisphere to approve same-sex marriages.

“Today’s landmark ruling makes clear once again that DOMA is a discriminatory law for which there is no justification. It is unconstitutional for the federal government to create a system of first- and second-class marriages, and it does harm to families in Massachusetts every day,” she said.

“All Massachusetts couples should be afforded the same rights and protections under the law, and we hope that this decision will be the final step toward ensuring that equality for all,” Coakley said in a statement.

Appeals Court Judge Michael Boudin wrote the unanimous decision for a three-judge panel.

“Invalidating a federal statute is an unwelcome responsibility for federal judges; the elected Congress speaks for the entire nation, its judgment and good faith being entitled to utmost respect,’’ he wrote. “But a lower federal court such as ours must follow its best understanding of governing precedent, knowing that in large matters the Supreme Court will correct mis-readings.”

Boudin noted that if DOMA is left intact, then same-sex couples legally married in Massachusetts are being denied federal benefits routinely provided to heterosexual couples. That, he said, cannot withstand legal scrutiny.

“Under current Supreme Court authority, Congress’ denial of federal benefits to same-sex couples lawfully married in Massachusetts has not been adequately supported by any permissible federal interest,’’ he wrote.

“To conclude,” he added, “many Americans believe that marriage is the union of a man and a woman, and most Americans live in states where that is the law today. One virtue of federalism is that it permits this diversity of governance based on local choice, but this applies as well to the states that have chosen to legalize same-sex marriage.’’

While saying that gays and lesbians have been victims of chronic discrimination, Boudin also said the push for the law was not mainly motivated by anti-homosexual fervor.

“As with the women, the poor and the mentally impaired, gays and lesbians have long been the subject of discrimination,’’ he wrote. “In reaching our judgment, we do not rely upon the charge that DOMA’s hidden but dominant purpose was hostility to homosexuality. The many legislators who supported DOMA acted from a variety of motives, one central and expressed aim being to preserve the heritage of marriage as traditionally defined over centuries of Western civilization.’’

The court issued its ruling in two related cases. In the first, a lawsuit was brought by seven same-sex couples married in Massachusetts and three surviving spouses of those marriages. The state brought a companion case, arguing that the federal government had historically left the definition of marriage to the states, even when the states disagreed on controversial issues such as interracial marriage.

A federal judge in Boston ruled in 2010 in favor of both Massachusetts and the same-sex couple plaintiffs.The Obama administration has said it will not defend DOMA in the courts. But the Republican-led US House of Representatives stepped in to press the Circuit Court appeal. And it is now likely to take the case to the US Supreme Court, said GLAD’s Bonauto.

Paul Clement, an attorney for the House Bipartisan Legal Advisory Group, which the House set up to defend DOMA, did not immediately return a call for comment.

Jonathan Knight and Marlin Nabors, two of the GLAD plaintiffs in the case, who have been married for the past five years, said in a telephone conference call organized by the organization that they were happy with the decision.

“Marlin and I have been together for seven years and married for five years,’’ said Knight. “We do our very best to live our lives together in truth and authenticity and we know that our relationship is full of the same kind of hopes, struggles, and dynamics as all other couples.’’

He added, “We are thrilled to learn that the law is on the same page, and that we fall into the same class as everyone else. We are no longer second-class citizens.’’

The Defense of Marriage Act was enacted during President Clinton’s administration, at a time when Congress was wary of a Hawaii court decision that could have enabled that state to recognize same-sex marriages. Hawaii lawmakers later restricted marriages to unions between a man and a woman, though lawmakers gave same-sex couples some civil protections.