Child porn is the emotional argument, the only possible case where most people might agree that some content can be illegal. A more representative scenario is illegal copies of music or a political pamphlet in China.

Open questions :

1) If childporn was not banned, how many people would consume that content anyway?

2) What about hyper-photorealistic childporn movies with fictional characters and future rendering technologies? How will you tell the video is true or not and worth to be removed?

Technological progress will not stop because some people abuse the technology.