Censorship is the first step towards authoritan rule and loss of freedom of speech and expression. In this day and age , where diversity is (supposedly) tolerated and looked forward to , censorship of information is basically the exact opposite.
Read George Orwell's 1984 , you'll be able to draw some conclusions for yourself.
Besides, why would you need to ban pornographic if you can use filters for software , like browsers for example.
Governments would make the worst possible step if they try to ruin the most open and free media of all. A good example for that is China or North Korea.