Reminds me of a good article I read a few weeks ago, about censorship creeping into comedy -
here if you're interested.
The problem is always, as has been highlighted, where you draw the line. If you censor one joke, just like if you censor one word, what else should you censor? Even when you've decided which words are acceptable, meanings change, and less exposure makes things more exotic and interesting, compelling people to find them out and use them, and thus further increase their controversy and the level of censorship.
Parents and their children are an obvious point of reference in the argument against. There will be things that children cannot comprehend and shouldn't be exposed to, yes, but they aren't the sort of things you tend to stumble upon, and again as has been stated there are web filters of various intensities; anything else is already in the public domain, and it being acceptable for adults, and there being so many adults, children will pick things up. If material is censored it is swept under the rug, it is controversial in some way or another, and so it will not be readily discussed; not being readily discussed will lead to understandings being sought from other sources, or not sought at all - this is far more dangerous than learning about them in a 'controlled environment'. The protective parental environments, by which some children will know these things and some won't, are dangerous for forming opinions and morals, and for the child's well-being amongst peers; if you don't get a clear, early understanding of good and bad things, if you don't have the knowledge of the two extremes, the two sides of humour or language or human/animal behaviour or human depiction, how can you make reasonable judgements? Linked into this is the idea that something is censored essentially because is it bad or wrong - why is nudity wrong? Why is it wrong to say a certain word? Because something gains certain connotations, society as a whole decides to surrender it rather than reshape it.
Society naturally limits certain controversial materials as a result of demand, itself as a result of morals. This is key. Parents teach morals in what they do and what they say, not necessarily in the sense of bringing their child up within a religion etc, but in what their actions tell their children. Removing all censorship won't lead to mainstream child pornography, snuff etc because there would not be enough demand; there is demand for so called swear/curse words and pornography because a majority of people enjoy these things, and everyone should be able to learn why and make their own judgement, based on the context presented to them by their upbringing.
Sorry if this is broken and convoluted, but it's gone 2am. :P