Those things are not illegal. That is the whole point. They get pressure for things that are legal.
Incest is even legal in real life in most places. How on earth is banning fiction about a legal thing in any way reasonable.
There is also a different discusssion as how certain countries have very vague and dubious laws about "obscenity" and forbidding drawn pictures of boobies. I exagarrate, but banning fiction is digital book burning. And yes, even cp is legal in countries that do not have such dubious laws, provided it is fictional and non realistic looking. And let's not delve into all the other fetishes that are best kept in fiction and not enter reality. Fiction is there for a reason. To explore concepts that are impossible, illegal, or simply unrealistic - like the typical avn where the hero gets all the women and none of the women has an issue with that.