Wow - what a tricky subject. The net cannot function with censorship. One person's censorship is another person's clipping service.
Censorship is what some people decide is good for others without their consent.
OK, nobody says you must carry alt.sex.bondage. The only way to check that is to censor all mail. You'll have to ban encryption too. Not the people who like pornography - they find alternatives, of which there are plenty. The losers are lawyers fighting the government, human rights activists, basically people whom the government does not like.