(Reuters) - A key witness for lawyers seeking to defend California's ban on same-sex marriage in federal court in 2010 has changed his view on the subject, and pronounced his support for giving gay unions social recognition.

David Blankenhorn, founder of the conservative Institute for American Values think tank, wrote in an opinion piece for the New York Times that he now believes the time for "denigrating or stigmatizing same-sex relationships is over."

"Whatever one's definition of marriage, legally recognizing gay and lesbian couples and their children is a victory for basic fairness," Blankenhorn wrote in a piece published on Friday.

In 2010, Blankenhorn was the final witness called to defend California's ban on gay marriage, which was passed by voters in the state in 2008 in a ballot measure called Proposition 8.

Blankenhorn began his testimony by asserting that the best environment for children is to live in a house led by a man and a woman. "The optimal environment for children is if they're raised from birth by their own natural mother, who is married to their own natural father," Blankenhorn told the court.

But in a surprise to observers of the trial, Blankenhorn seemed to concede certain points to gay marriage advocates, under persistent cross-examination from veteran litigator David Boies, who helped launch the legal challenge to Proposition 8.

Blankenhorn said on the witness stand he believed "adopting same-sex marriage would be likely to improve the well-being of gay and lesbian households and their children."

In his New York Times opinion piece, Blankenhorn maintained gay marriage "has become a significant contributor to marriage's continuing deinstitutionalization."

"I have written these things in my book and said them in my testimony, and I believe them today," he wrote in the piece. "I am not recanting any of it."

But Blankenhorn went on to argue that he has changed his view in consideration to others, due in part to an "emerging consensus" in which the public has come to believe gay marriage is about accepting gays and lesbians "as equal citizens."

"And to my deep regret, much of the opposition to gay marriage seems to stem, at least in part, from an underlying anti-gay animus," Blankenhorn wrote. "To me, a Southerner by birth whose formative moral experience was the civil rights movement, this fact is profoundly disturbing." Continued...