What the parent is getting at is it's not a mystery, such definitions already exist in all kinds of jurisdictions.
In any case it's trivial to come up with such a definition that covers most cases. Doesn't matter if it doesn't cover some gray areas or 100% of it. Laws can be supplemented and ammended.
We don't have an all-encompassing definition of porn either, but we have legal definitions, and we have legal frameworks regarding it.
> Doesn't matter if it doesn't cover some gray areas or 100% of it.
That's exactly the thing that matters when you're dealing with something where every loophole is going to have a caravan of trucks driving through it.
> We don't have an all-encompassing definition of porn either, but we have legal definitions, and we have legal frameworks regarding it.
You're picking the thing which is a hopeless disaster as your exemplar?