Pornography has been illegal China since the People's Republic was founded in 1949. However, in 2010, the government eased up and stopped censoring explicit adult content on the Internet. No one knows why the ban was lifted. Some reasoned that the government had bigger things to focus on like the protests that took place on the anniversary of the 20th anniversary of Tiananmen Square. So, in summary: Any sites about government officials have affairs with 18-year-olds are blocked, but Pornhub is all good.
Oddly Enough, Explicit Adult Content is Not (Completely) Censored