IN THE UNITED States sex is a private activity, not only by tradition but also by law. Consenting adults are free to engage in sexual activity in private, but they are forbidden by law to do so in public. Laws against public nudity and lewd behavior also prevent people from performing sex acts onstage for the entertainment of others.

What happens, however, when such acts are performed before a camera? Does an act that is illegal onstage become legal because it is presented to the public in the form of photographs, a motion picture, a video, or a live webcam feed? The makers and sellers of sexually explicit materials, known as pornography, believe so. They maintain that since pornography is published, it is protected by the First Amendment. Others disagree. Publication does not magically transform illegal acts into legal ones, they argue. Pornographers must obey the...