It's an interesting edge case in practice, with many different legal stances around the world.
In the USA, the potential problems associated with such material, largely do not outweigh free speech rights, unless the Miller Test for obscenity is crossed. That free 'speech' rights offer legal protection for child sex robots, was found so widely disturbing it led to the Curbing Realistic Exploitative Electronic Pedophilic Robots (CREEPER) Act, which even so only banned import. The 2003 PROTECT Act banned fictional depictions, but, free speech can be used as a defence in many cases. It's described as a 'gray area' country on these laws.
Japan has a more-or-less total ban on depictions of genitalia, but basically no other constraints on artwork and comics. Manga in Japan has always dealt with adult themes and was largely aimed at adult audiences. It's interesting to look at the history of that. In a society with very little sex education, manga has perhaps been of extra significance to teenagers. They only banned making child porn in 1999, and possession in 2015, so they have long been an outlier in tolerance of these issues. Fictional child sex images are completely legal there still.
The UK has maintained very restrictive laws, with 'hardcore' porn depicting penetrative sex never having been legal to make in the UK, & only for sale in sex shops and online. A widely criticised 2009 law on 'extreme pornography' made a host of vague provisions which cannot be easily interpreted, & explicitly banned non-photographic style images of minors.
These different cultures have specific hierarchies of moral priorities, and different historical processes have acted to shape policies. A lot of policy comes down to vague wording, and a sense that courts will 'know it when they see it', mirroring sodomy laws which historically were so vague we still don't know what many states actually prosecuted. The threat of public disorder, like from puritans or religious conservatives in Pakistan Afghanistan or Iran, shows how failure of governments to keep policy up to speed with public sentiment can lead to unrest, even revolution. Movement towards more lenient laws has mostly been about specific cases, like the many legal cases around Lady Chatterley's Lover, and intent is very important - eg art vs titillation.
Why governments and media get very concerned with sexual behaviour, is interesting. We discussed it here: How do ethicists tackle the question "Is it immoral to have sex in public places?" Is it possible to use rational and empirical ideas to answer? Shame and disgust seem to be of very high importance in rapidly reconstructing the functioning of societies. Through cultural mechanisms, we seem to have become able to access these extremely powerful biologically evolved shapers of behaviour.
Are fictional images immoral? It is only a very recent consensus to think so, more led by disgust than proof of harms. The 'moral matrix' of a society, what order it puts moral priorities in, and how sensitive to infringement, and how universally laws are enforced, are deeply cultural - a lot of what we take for granted now is newer than we think, eg prohibiting marital rape. So I'd suggest in this case, don't look to acts themselves, which here can't be fitted into the normal moral methodology of direct harms. Instead, consider what your community think. Ultimately, that has always been of higher importance in moral reasoning, than legal consequences. Self-censorship, has always been more powerful than censorship.
It important to recognise that moral judgement is not simply a matter of decrees or commandments, or direct intuitions; but of culture. Philosophers should pay more attention to the processes of moral reasoning on new and disputed topics, where the actual mechanisms of personal and community deliberation about moral behaviour and attitudes are revealed in practice.
