http://www.supremecourtus.gov/opinions/01pdf/00-795.pdf The Supreme Court rules that fake child porn is protected speech. It's fifty pages or so but it's good reading.
In response to the argument that fake child porn should be prohibited because it is difficult to distinguish from real child porn (thereby making it difficult to prosecute instances of real child porn), the court—disagreeing—notes: “The Government may not suppress lawful speech as the means to suppress unlawful speech. Protected speech does not become unprotected merely because it resembles the latter.”
Clarence Thomas—though he voted with the majority—wrote a separate opinion in which he questions this argument. Thomas: “if technological advances thwart prosecution of ‘unlawful speech,’ the Government may well have a compelling interest in barring or otherwise regulating some narrow category of ‘lawful speech’ in order to enforce effectively laws against pornography made through the abuse of real children.”
I think that this is largely right: it's sometimes necessary to prohibit or control a certain activity or product not because it's inherently wrong, but because they're easily confused with something that is. It's not inherently wrong to wear a helmet into a bank, for example—banks prohibit it though because it's impossible to distinguish the helmet-wearing motorcyclists from the bank-robbing kind. (A tax is levied on blank media (at least cassette tapes) for a similar reason.) 18:04