“Now a few family-law scholars have begun to argue that there is nothing special about the number two—if three or four or five adults have a parental relationship with a child, the law should recognize them all as parents. Going beyond two, these scholars argue, would better reflect the dynamics of the modern family, and also protect the children in such families. It would ensure that, even in the event of a split or major disagreement between the adults in question, the children would not be deprived of the affection, care, and financial resources of any of the people they have grown up regarding as their mothers and fathers.”