Opponents counter that the issue is complex and that allowing multiple parents in one section of law inevitably raises questions that could spark litigation in other sections.
Tax deductions, citizenship, probate, public assistance, school notifications and Social Security rights all can be affected by determinations of parenthood, notes the Association of Certified Family Law Specialists.
"This bill, in our opinion, if passed, will cause significant unintended consequences," said Diane Wasznicky, the group's president and a family law attorney in Sacramento.
Assemblyman Donald Wagner, an Irvine Republican who opposes SB 1476, noted it could spark litigation, say, in a case of a wrongful death of a child with four potential parents and determining who has a claim.
Karen Anderson, of the California Protective Parents Association, said the legislation could result in a child being bounced among multiple adults in a bitter family breakup.
"It's hard enough for children to be split up two ways, much less multiple ways," she said.
Attorney Catherine Sakimura of the National Center for Lesbian Rights, a co-sponsor of SB 1476, said judges would be required under the bill to consider a child's stability in awarding custody and visitation.
SB 1476 states that concerns about child stability "may mean that not all parents share legal or physical custody."
Wasznicky counters that it makes no sense to declare someone a parent, essential to a youngster's well-being, but then "cut that person out" of the kid's life.
"Either someone is vital enough in a child's life to be labeled a 'parent' and have certain rights and obligations to the child, or the person is not," she contends.
Ellen Pontac, a Davis gay-rights activist, said she and her wife, Shelly Bailes, each had two children when they began their relationship 38 years ago. She understands how someone can function as a child's parent but accrue no legal rights. Government should accommodate changing times, she said.
"I just think that people should be able to create their own lives," she said.