There's a great wikipedia page on this very topic [1]. In some countries like Germany fictional porn isn't considered porn, while other countries like Australia or France consider the possession of drawings of naked minors a crime worthy of jail sentences. And then there's the US where having the images on a computer is fine, as long as they aren't sent over the internet and the computer never crosses state lines.
In general it's a topic people are careful about because the legal situation is complex, and in many countries associated with the potential of heavy jail sentences.
When you stop and think about it, it is kind of insane that a fictional drawing in private possession, not even communicated to anyone, is already sufficient to land someone in prison in so many jurisdictions. Isn't this a clear case of legislating moral purity (as opposed to actual harm)?
In general it's a topic people are careful about because the legal situation is complex, and in many countries associated with the potential of heavy jail sentences.
1: https://en.wikipedia.org/wiki/Legal_status_of_fictional_porn...