Convicted child pornographer Brian Dalton was charged with pandering child pornography on the basis of his private writings in a personal journal in his home. He pled guilty, which normally precludes appeal and may prevent me from collecting on my bets.
Point one: The states can do anything that isn't prohibited by the Constitution. (This elusive concept is admittedly difficult to grasp, especially if you are a Supreme Court justice and prefer to think of yourself as "Czar of the Universe.") If a state wants to outlaw artichokes, it can, unless the artichoke is actually, say, a gun, in which case it is constitutionally protected.
Thus, the only question is whether Dalton's private journal is protected by the First Amendment.
Dalton insists he had no intention of sharing his journal with his pederast friends. It was for his eyes only. This point has great emotional appeal, but throws into doubt whether Dalton's journal qualifies as "speech." To whom was he speaking? The reason burning an American flag is protected "speech" is that the First Amendment protects communication, not mere words.
If Dalton's journal was intended solely for his own individual pleasure, it's not apparent why it should have any greater constitutional significance than a blow-up doll. The whole point of the First Amendment is communication -- expressive content, the marketplace of ideas, the government cannot distinguish truth from falsity, blah blah blah.
It may seem intuitively correct that you have a right to talk to yourself, but it also seems intuitively correct that you have a right to artichokes. Unless the Constitution protects it, states can ban it. Dalton was either pandering child pornography or he was talking to himself -- which isn't obviously protected by the Constitution.
But let's say talking to oneself does constitute "speech." Not all speech is protected by the First Amendment. Obscenity, for example, is not protected.