It is easy to interpret the popularity of a network television series like “Modern Family” as proof that we have mainstreamed the various and sweeping ways domestic life has reshaped itself over the past two decades. A nation of squares would not embrace a comedy about a badly dressed, middle-aged gay couple raising an adopted Vietnamese baby, we tell ourselves, no matter what they might say in Copenhagen or Berlin.

Gay rights are moving forward; single women now account for 41 percent of all births. Americans build caring families with lovers, friends and neighbors; from one-night stands and anonymous providers of genetic material. And yet, even in a place as progressive as New York, the legal system has been slow to synchronize to these altered realities.

It is hard to imagine anyone experiencing this more viscerally right now than a man named Jonathan Sporn, a 54-year-old pharmaceuticals executive living on the Upper West Side, who in a sense has fallen prey to a system that excessively privileges the conventional family models from which there seems to be a growing exodus.

According to a custody petition Dr. Sporn filed in Manhattan Supreme Court last month, he and his girlfriend, Leann Leutner, had a baby boy — Lincoln Amory Aurelian Sporn Leutner — last July, with the help of in vitro fertilization. The couple had issues conceiving but found success with the use of an anonymous sperm donor. Dr. Sporn and Ms. Leutner, both of whom had divorced previous partners, were not married. But they had lived together since 2010 and were deeply committed to starting a family.

Then in mid-December, Ms. Leutner, a lawyer at Simpson Thacher & Bartlett, left with Lincoln for New Jersey, where a few days before the end of the year she got a new apartment. On New Year’s Day, she committed suicide. Her death, according to the petition, followed previous attempts to take her own life and a long history of psychological difficulty made worse by postpartum depression. Her departure for New Jersey had been preceded by a stay at Mount Sinai for psychiatric treatment. Ms. Leutner left the hospital prematurely, the petition states, against Dr. Sporn’s judgment, assisted by a friend.

Ever since his mother’s death, the baby has been in the custody of child protective services. He is currently in foster care in New York City, even though Dr. Sporn desperately wants him returned home and Ms. Leutner’s sister, Susan Sylvester, who lives in Illinois, is also seeking custody.

How could such a strange state of affairs have come to pass? In a proceeding on Tuesday, Justice Laura E. Drager acknowledged that mandated visits to both homes determined that either would be suitable for Lincoln. She also said it was in the best interest of the child, and the city, to settle the matter quickly. And yet, Lincoln technically occupies the Dickensian status of “destitute,” a term new to New York State family statute pertaining to children who have no known parents. The next hearing on the child’s fate is scheduled for March.

Dr. Sporn’s petition offers a moving portrait of his investment in fatherhood, outlining the joy he took in his son’s birth, the willingness with which he changed diapers and bathed the baby. He found himself falling “deeper and deeper in love” with the child. “I looked forward to his cries in the night just to have another opportunity to hold this child in my arms and soothe him back to sleep,” he says in court documents.

But from the perspective of the law, a parent in Dr. Sporn’s situation is effectively not a parent at all. He was not married to Lincoln’s mother. He has no blood relationship to the child. And he did not take steps to legally adopt him after his birth.

The law doesn’t reflexively recognize the role you have played, or the obvious parental intent that attaches to anyone who has gone to the trouble to have a child with assisted reproductive technology, or the number of times you’ve performed 3 a.m. feedings. Dr. Sporn’s case resembles the kind gay parents frequently find themselves in when they split up: a mother or a father who is not biologically related to the child typically has a difficult time gaining recognition as a parent in court if formal adoption proceedings haven’t already been started.

Legal precedent itself is confusing. A few years ago, Susan Sommer, the director of constitutional litigation at Lambda Legal, won a case for a client who sought to be regarded as a legal parent to a child, not genetically her own, whom she had with a female partner. The couple previously had a civil union in Vermont, and the court in New York agreed to comply with Vermont law regarding parental rights. But the decision did not overtly overturn a New York precedent, more than 20 years old, that determined a lesbian mother unrelated to her child was a legal stranger. Yet genetics aren’t always paramount either. In a 2006 New York City case, a man was helping to raise a child he thought was his. After it was revealed that another man was the biological father, he was still held responsible for child support.

“There was a time when being ‘illegitimate’ afflicted a whole array of legal disabilities on a child,” Ms. Sommer said. “The law had to, as a constitutional matter, adjust.” And it started to: a series of United States Supreme Court cases in the 1960s and ’70s sought to protect children from disadvantage if their parents were unmarried.

But there is still much ground to be covered. We’re watching “Modern Family,” but certain dimensions of the legal system have yet to change the channel from the era of black and white.