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There is “no rational reason” to deny federal benefits to gay couples who are already married under state law, said Mr. Ball, who is openly gay and has written several books on gay issues.

A clear example, he said, is Edith Windsor, an 81-year-old lesbian who in 2009 lost her lifelong companion and Canadian spouse, Thea. Their marriage “was recognized by New York, but not by the IRS,” he said, and since Ms. Windsor could not claim a marital tax deduction for their estate, she was forced to pay more than $350,000 on it.

Arguments about procreation or optimal settings for child rearing “do not help the government defend the rationality of this type of unfair, unjust and, quite frankly, un-American treatment of married couples,” said Mr. Ball, noting that Ms. Windsor, with the help of the American Civil Liberties Union, filed last year the New York lawsuit against the United States.

Other witnesses said the Obama DOMA decision was misguided and stemmed primarily from an effort to cater to gays, a favored special-interest group.

The executive branch knows that, except in rare instances, it is expected to vigorously defend congressional laws, said Edward Whelan, president of the Ethics and Public Policy Center and an specialist on constitutional law.

By refusing to defend DOMA, “the Obama administration has subordinated its legal duty to its desire to please a favored and powerful political constituency,” he said.

Marriage means “the union of husband and wife for a reason,” said Maggie Gallagher, author and co-founder of the National Organization for Marriage. “These are the only unions that create new life and connect those children to their mother and father.”

The governments interest in supporting these vital unions is so deeply embedded in law that the U.S. Supreme Court held in 1888 that without marriage, “there would be neither civilization nor progress,” she said.

Same-sex marriage does not fit with this definition of marriage, she added. Instead, it leads to such a radical transformation of the public meaning of marriage that — as is now happening in Canada — it invites legal recognition of other kinds of relationships, such as polygamy, said Ms. Gallagher.

In addition to the ACLU-led Windsor lawsuit, DOMA is the subject of separate legal actions brought by groups such as Gay & Lesbian Advocates & Defenders, Lambda Legal and Legal Aid Society in San Francisco.

The plaintiffs generally are gay couples seeking federal benefits after legally marrying in a U.S. state or in another country.

DOMA was written with the help of constitutional law specialists as the gay-marriage issue was heating up 15 years ago; it passed with overwhelming bipartisan support and was signed by President Clinton.

In addition to defining marriage under federal law as the union of a man and a woman, DOMA tells states they do not have to recognize another states same-sex unions, as normally happens with marriages now under the U.S. Constitution’s full faith and credit clause.