Such a law might serve not only same-sex families, but adoptive ones as well, where there may be a relationship with a biological parent.
However, Adam Pertman, executive director of the Evan B. Donaldson Adoption Institute, said situations where the law might be applicable are "pretty limited."
"Most people don't aim for this and don't need it," he said. "It's an arrangement that's created for specific circumstances -- but I don't see a big trend here."
"People in the adoption world get very concerned about a law like this," said Pertman. "One of the concerns they have about open adoptions is co-parenting and it simply is not. There are circumstances where there is a real need and individual cases where it serves the needs of the child. That should be the focus, to have a law that permits the child to get what he or she needs."
And some legal experts in California question the impact of such a law on an array of issues like tax deductions and wrongful death suits.
Leno acknowledges that the law might be applied in "rare circumstances" and only when it is required "for the best interests of the child."
"Some of the hyperbolic corners of the opposition are suggesting there could be four, six or eight parents," he said. "But I think that it will not be used when a child has too many parents, but when there are too few."
The bill was co-sponsored by the University of San Diego School of Law's Children's Advocacy Institute and the National Center for Lesbian Rights.