Once again, California is taking the lead in recognizing the realities of modern-day families while ensuring that the best interests of the child remains paramount. This is sure to cause the heads of some religious fanatics and same-sex marriage opponents to spontaneously combust:

Bill Delaney’s two little girls spend three nights a week with their fathers, at the home Mr. Delaney shares with his husband in San Francisco. The other nights, they stay with their mothers, a lesbian couple who live nearby. The girls have four parents — a result of a kind of nontraditional family arrangement that has become increasingly common. But officially, California, like most other states, recognizes no more than two legal parents. That limit could soon be lifted.

A bill moving through the California Legislature would allow judges to recognize more than two legal parents for a given child, opening the door for alternative families to seek legal recognition of their relationships. “There are literally scores of different families and circumstances,” Mark Leno, the state senator who sponsored the bill, said. “This is about putting the welfare of the child above all else,” he said.

As fewer children are raised in traditional two-parent households, Mr. Leno’s bill has moved California to the center of a growing debate over how — and whether — such alternative family arrangements should be legally recognized. In just the last few years, Delaware and the District of Columbia have passed laws that allow for third “de facto parents,” who have the same rights and responsibilities toward their children as adoptive parents.

Meanwhile, courts in at least a half-dozen states, stretching from Oregon to Massachusetts, have also recognized three legal parents in a handful of cases over the last decade. Courts in Canada and New Zealand have as well. Nancy Polikoff, a professor at the American University Washington College of Law, said those cases merely allowed the law to reflect the many different kinds of families that already exist in the 21st century. “This is about looking at the reality of children’s lives, which are heterogeneous, as opposed to maintaining a fiction of homogeneity,” Professor Polikoff said. “Families are different from one another. If the law will not acknowledge that, then it’s not responding to the needs of children who do not fit into the one-size-fits-all box.”

Conservative groups, however, have called the California bill an attempt to redefine the family radically, opening the door for children to have six, eight, even a dozen parents. “This bill is a Trojan horse for the same-sex-marriage agenda,” Peter Sprigg, a senior fellow at the Family Research Council, said. “Advocates for same-sex marriage are very interested in separating parentage and marriage from biological parentage, because that’s the one thing same-sex couples can never achieve,” he added.

The bill passed the California Senate on a party-line vote, with Democrats supporting it and Republicans opposed, and could reach the Assembly floor by next month. Mr. Leno, a Democrat from San Francisco, insists that fears of children with dozens of parents are unrealistic. Third parents would still have to meet the same legal standards currently in place. A judge would also have to rule that it was in the child’s best interest to recognize a third parent. Instead, Mr. Leno said he hoped that allowing the court to recognize a third or fourth parent could help keep children out of foster care in a small number of cases.

Mr. Leno’s bill follows a court case here last year in which a young girl being raised by two lesbian mothers was sent to foster care after a fight landed one mother in the hospital and the other in jail. The girl’s father, who had maintained a relationship with his daughter, asked the court to release her to his custody. The trial court ruled that both mothers and the father were all parents. But a California appeals court reversed the decision, ruling that the child could have only two parents.

Still, parents hoping to keep their children out of foster care are hardly the only ones who hope to benefit if the bill becomes law. In San Francisco, Mr. Delaney said, the two sets of parents go on vacations together with the children, have family dinners together and have signed agreements about how to handle situations like a falling-out. But Mr. Delaney, who is the biological father of his two girls, is not currently one of their legal parents. He said his becoming a legal parent would give the whole family a greater feeling of security.

“This would be the final piece, so we don’t have to worry if something happens to the legal parents or if I am out with the kids and something happens,” he said. “Legally, they could just take my kids and I couldn’t do anything about it.” And while many of the third-parent adoption cases have involved same-sex couples, heterosexual couples have also sought legal recognition for third parents.

If the past is any guide, opponents will soon be arguing that this law will ultimately permit a child to have a goat, ostrich or warthog as a parent. Just watch and see….