The pro-censorship stance asserts that some information should not be available to some people, this being to protect them. There is no denying that some information has the potential to be misused (meaning "used to the detriment of others"). However, this does not mean that censorship is desirable. Indeed, censorship won’t help, as prohibition doesn't work, and introduces a whole range of new problems.
Further, to focus on censorship is to miss the wider issue: information, as with money, technology, political influence and physical strength, is just another road to power. People misuse power all the time - censoring information will just make the ignorant more vulnerable. The wider issue is thus not how to control information, but rather, how to encourage the responsible use of power.
"Oh, it is excellent to have a giant's strength, but it is tyrannous to use it like a giant." -- Shakespeare
At the heart of the issue lies a basic lack of respect for others, which stems from ignorance. If people better understood each other, conflict would be reduced.
Ironically, it is communication that relieves ignorance and fosters understanding. Relationships are created, maintained and terminated with communication. Relationships foster understanding by facilitating cultural exchange. Communication thus fosters respect for others. This respect translates to tolerance and benevolence, and thus encourages the responsible use of power.
Censorship inhibits communication and thus discourages the responsible use of power. Consequently, censorship, far from "protecting" people, exacerbates the problem, by denying it. In this sense, censorship can be said to encourage the irresponsible use of power.
"Peace cannot be kept by force. It can only be achieved through understanding." -- Einstein
Remove the "need" for censorship by empowering the audience to view information objectively and make rational decisions to accept or reject the opinions the information contains. This empowerment need not be accomplished by regulation - simply opening the floodgates will make the audience realise the scope of views "out there" and, with new-found tolerance and understanding, realise that no particular view is superior, that all views are equal, and all views thus deserve equal respect.
However, unrestricted expression is a two-edged sword. Being unprepared for expression that might be offensive is abusing the privilege. It is the responsibility of the audience to be aware that offence might be imminent (caveat emptor), and react with similar distaste to sour milk - spit it out.
definitions and assumptions
It has been suggested that the artist has some responsibility to warn the audience. This argument relies upon artist and audience thinking similarly - when an artist warns that the upcoming show might be "offensive to some viewers", what exactly is meant by offensive? For the warning to be meaningful, the word "offensive" must be defined.. the artist would end up fielding more questions on the definition than they would expressing their artform. Defining "offensive" is impossible - such a concept is subjective, and a common definition acceptable to all parties will never be achieved.
Avoiding detailed definition, while still offering a classification strategy, is assuming that the artist and audience think similarly: that there is a shared definition of what "mature" (as in "for mature audiences only") means. This is a tenuous assumption, given the art in some galleries and the mindset of some audiences. It does not remove the dependence upon subjectivity and is thus susceptible to failure - some are offended easily, while others are offended at nothing.
It does not matter whether a warning is given or not - the words are meaningless and unlikely to be useful.
Thus, warnings ahead of time introduce additional subjectivity, in the form of semantics and association with other such anticipated expressions, and may in fact reduce or negate the effect of the expression as a result.