IN my 2007 book, “The Future of Marriage,” and in my 2010 court testimony concerning Proposition 8, the California ballot initiative that defined marriage as between a man and a woman, I took a stand against gay marriage. But as a marriage advocate, the time has come for me to accept gay marriage and emphasize the good that it can do. I’d like to explain why.

I opposed gay marriage believing that children have 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. I didn’t just dream up this notion: the United Nations Convention on the Rights of the Child, which came into force in 1990, guarantees children this right.

Marriage is how society recognizes and protects this right. Marriage is the planet’s only institution whose core purpose is to unite the biological, social and legal components of parenthood into one lasting bond. Marriage says to a child: The man and the woman whose sexual union made you will also be there to love and raise you. In this sense, marriage is a gift that society bestows on its children.

At the level of first principles, gay marriage effaces that gift. No same-sex couple, married or not, can ever under any circumstances combine biological, social and legal parenthood into one bond. For this and other reasons, gay marriage has become a significant contributor to marriage’s continuing deinstitutionalization, by which I mean marriage’s steady transformation in both law and custom from a structured institution with clear public purposes to the state’s licensing of private relationships that are privately defined.