The answer to the question depends on what country you are in. If there are different criminal codes for any subdivisions of said country (i.e., the states in the USA, India or Brazil), it depends on the criminal code of said state. In Denmark, where I live, it is not illegal to draw or paint pictures ((or create computer generated animations etc.) of minors engaging in sexual activity (provided you're not doing it from live models, of course!), nor to write about it, but is is highly illegal to make videos or photos (and esp. distribute them) of minors (i.e. real persons) engaging in sexual activity.
The reason for this is that the law protects minors from actual abuse, but it doesn't punish "dirty thoughts" that are not carried out into action; you can't be punished for your thoughts or for expressing them (this is a result of there being freedom of speech and expression...).
In the same way, it's ok to write about, or make a cartoon or a computer animation which depicts or shows people being robbed, assaulted, murdered or tortured , while actually robbing, assaulting, murdering or torturing people is of course, a criminal and punishable act.
So it depends on what country/state you're in, and if your server is hosted in say, Denmark, your artwork accessed there would only be subject to Danish law.
Edit: I am constantly baffled by US laws. Why is it that everything in the US regarding weapons and violence is legal, while everything pertaining to sex is illegal? What happened to "make love, not war"?