It's something that Japan is behind the times on banning.
Limiting the freedom of speech is not something that should be considered progress, especially limiting the ability to create art and fiction.
If there are explicit depictions of sex in them, it would count as child pornography by US law.
No, it isn't, Ashcroft vs Free Speech Association struck down that provision, the only virtual pornography that is illegal in the us is that which is indistinguishable from real child pornography.