Over the years the federal has prosecuted many pornographers under obscenity laws. These are first amendment exceptions the Supreme Court made in 1973. Essentially it means sexual material can be banned if it's offensive enough, and has no serious literary artistic, political or scientific value.
This law has been used repeatedly to prosecute people who make pornography with consenting adults and it needs to end. Consenting adults have a right to make pornography and to perform sexual acts. This exception is outdated, an infringement on civil liberties and needs to go away fast.