David Blankenhorn, president of the New York-based Institute for American Values, asserted in an op-ed essay published in the New York Times today that he now supports allowing gay and lesbian marriage.
"Whatever one's definition of marriage," Blankenhorn wrote, "legally recognizing gay and lesbian couples and their children is a victory for basic fairness."
Blankenhorn was the main witness called by supporters of Proposition 8, California's voter-approved ban on same-sex marriage, at a trial in U.S. District Court in San Francisco in 2010.
The trial was held on a constitutional challenge to the 2008 voter initiative by two same-sex couples.
Later in 2010, U.S. District Judge Vaughn Walker struck down Proposition 8, and this year, the 9th U.S. Circuit Court of Appeals also found the measure unconstitutional, although on narrow grounds that would apply only to California.
The sponsors of Proposition 8 are now appealing to the U.S. Supreme Court.
At the trial two years ago, Blankenhorn testified he believed that marriage would benefit lesbian and gay couples and their children, but said he feared that allowing same-sex weddings would undermine heterosexual marriage as an institution.
In today's essay, Blankenhorn wrote he had hoped that debating gay marriage "might help to lead heterosexual America to a broader and more positive commitment to marriage as an institution."
"But it hasn't happened," Blankenhorn wrote.
"If fighting gay marriage was going to help marriage overall, I think we'd have seen some signs of it by now," Blankenhorn wrote.
Blanenhorn said he hopes that gay people and heterosexuals who want to strengthen marriage will form coalitions to work together.
Carla Hass, a spokeswoman for the sponsors of Proposition 8, said the sponsors had no immediate comment.
Hass said the sponsors have not yet filed their petition for review with the Supreme Court, but are going ahead with plans to do so.
The lawsuit challenging Proposition 8 was supported by the Los Angeles-based American Foundation for Equal Rights.
Adam Umhoefer, the foundation's executive director, said, "We are happy to consider Mr. Blankenhorn a supporter" in the quest for marriage equality "and applaud his commitment to equality."