The United Kingdom's Coroners and Justice Act 2009 (part 2, chapter 2) makes possession of images portraying fictional child abuse punishable by law. Wikipedia has a summary, but here's a snip from the primary-source legalese:
(5) “Child”, subject to subsection (6), means a person under the age of 18.
(6) Where an image shows a person the image is to be treated as an image of a child if—
(a) the impression conveyed by the image is that the person shown is a child, or
(b) the predominant impression conveyed is that the person shown is a child despite the fact that some of the physical characteristics shown are not those of a child.
Of particular significance is the conviction of anime fan Robul Hoque whose cartoons the court found to be infringing. As summarised by The Mirror's Gareth Lightfoot:
Despite being cartoons, they were classified as prohibited images as they depicted young girls, some in school uniforms, some exposing themselves or taking part in sexual activity.
The police, children’s charities and others represented on the Criminal Law Sub Group of the Home Secretary’s Task Force on Child Protection on the Internet are concerned that the fantasy images themselves fuel abuse of real children by reinforcing potential abusers’ inappropriate feelings towards children.
In addition, these images can be used to help groom victims. Whilst the use of these images for grooming purposes may result in a prosecution under the Sexual Offences Act 2003, making their possession an offence would enable these images to be taken out of circulation and make them unavailable for use as a grooming tool.
In ¶ 1 of the same section, the consultation also expresses a need for caution:
It is the case that cartoons, drawings and material created entirely by manipulation of computer software do not harm real children in the same way as taking indecent photographs of children, which are currently covered by legislation. The creation of a simple possession offence in respect of fantasy material is a serious step.
Australia has similarly decided to outlaw films where a naked actor "appears to look under the age of 18", regardless of actual age.
I suspect that such laws stem from a wrong working hypothesis of "fantasy leads to action". Regarding other human sexual behaviour, the contrary is supported by evidence that—
Does similar evidence exist for whether such a ban protects children from abuse?