David Blankenhorn, founder the Institute for American Values and a witness at the trial of the ballot which led to California’s gay marriage ban says he will no longer fight against marriage rights for gay couples.

Mr Blankenhorn wrote in an opinion piece for the New York Times that he still believed children had the right “insofar as society makes it possible, to know and to be cared for by the two parents who brought them into this world”.

But, he said, “there are more good things under heaven than these beliefs”.

Mr Blankenhorn cited firstly the “equal dignity of homosexual love” and said that while gay and straight relationships were not the same, “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.”

Secondly, he said “mutual acceptance” and “compromise” was a necessary part of living together in society, that “bending the knee a bit, in the name of comity, is not always the same as weakness” and that he wanted “conciliation”, not “further fighting”.

Thirdly, Mr Blankenhorn said the emerging consensus among the broader general public had persuaded him in part to change his views.

The consensus, he said “may be wrong on the merits”, but “surely it matters”.

He wrote: “I had hoped that the gay marriage debate would be mostly about marriage’s relationship to parenthood. […]

“In the mind of today’s public, gay marriage is almost entirely about accepting lesbians and gay men 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. To me, a Southerner by birth whose formative moral experience was the civil rights movement, this fact is profoundly disturbing.”

He continued: “With each passing year, we see higher and higher levels of unwed childbearing, nonmarital cohabitation and family fragmentation among heterosexuals. Perhaps some of this can be attributed to the reconceptualization of marriage as a private ordering that is so central to the idea of gay marriage. But either way, if fighting gay marriage was going to help marriage over all, I think we’d have seen some signs of it by now.”

Instead of campaigning against marriage for gay couples, Mr Blankhorn said he would “like to help build new coalitions bringing together gays who want to strengthen marriage with straight people who want to do the same”.

“For example, once we accept gay marriage, might we also agree that marrying before having children is a vital cultural value that all of us should do more to embrace? Can we agree that, for all lovers who want their love to last, marriage is preferable to cohabitation?

“Can we discuss whether both gays and straight people should think twice before denying children born through artificial reproductive technology the right to know and be known by their biological parents?”

Mr Blankenhorn had argued against gay marriage rights in the trial of Proposition 8 in 2010, characterising marriage more as a relationship in which to bring up children than a private relationship between two adults.

That court case ultimately ruled that Proposition 8, a public ballot through which voters revoked the rights of gay couples to marry in California, was unconstitutional. The court said his appearance constituted “inadmissible opinion testimony that should be given essentially no weight”.

It said: “Blankenhorn’s opinions are not supported by reliable evidence or methodology and Blankenhorn failed to consider evidence contrary to his view in presenting his testimony.”

Mr Blankenhorn’s Institute for American Values aims to ensure children grow up with both married parents, “renew the ethic of thrift”, combat “extremism in the Arab and Muslim world” and “civilise” public conversation.

He has previously acknowledged benefits of marriage rights for gay couples. In his book the Future of Marriage, he said it would promote stable, committed relationships for gay couples, mean fewer children would grow up in institutions, reduce the incidence of hate crimes and drive progress in the American dream of equal opportunity regardless of an individual’s circumstances.

Human Rights Campaign President Chad Griffin told AP: “While it can be difficult as a public figure to change course, I applaud him for taking a courageous and principled stand. His experience wrestling with the issue of marriage equality and coming out on the right side of history will be an inspiration to millions of fair-minded who are in the same place.”