In the 1980s, Alison D. and Virginia M., a lesbian couple who had lived together for three years in New York, decided to have a child. Together they chose the sperm donor, planned and participated in Virginia’s impregnation, and contributed equally in the financial and emotional responsibilities of raising the child, a boy. The women underwent the process again two years later when Alison became pregnant and gave birth to a girl. Six months later, when Virginia’s boy was 21/2 years old, the women separated.

For the next few years, Virginia allowed Alison to visit her son. When the boy was 6 years old, however, Virginia, the biological mother, barred Alison, the nonbiological mother, from further visitation. Alison sued to regain the right to visit the child she considered to be her son. She claimed that she had...