Well, that kind of law has come up again, and again, and again in the US, and every time it gets shot down in the Supreme Court because it violates the First Amendment since no actual victim exists, meaning the right to free speech takes precedent. That said, it's speech that's come under an increasing amount of fire as availability increases, and in nations without such clearly stated protections of speech (such as most of Europe, Australia and all your dictatorships) such material is banned. Furthermore, niche lobbying groups and international children's advocacy groups alike, both seemingly oblivious to the fact their time might be better spent thwarting the massive criminal networks that make and distribute child pornography (read up on wikileaks and weep for the world,) are increasingly itching to get lolicon/shota material banned in the two first world countries that have most lenient laws in regarRAB to drawn pornographic content depicting minors, the US and Japan.
Legally speaking though, with Supreme Court precedent repeatedly protecting that kind of stuff, it would take a major act of Judicial activism in the US (namely the Supreme Court, many of whom were on the bench when previous attempts at these laws were declared unconstitutional,) or a Constitutional Amendment to ban it. It'd be have to be a very specifically worded amendment as well, lest we accidentally ban written works from Shakespeare, Nabokov, Scooter Libby and countless others.
The way I see it: I dislike lolicons, shotacons and the content they make as much as the next guy, but just like the guy with the white supremacist bumper sticker and rebel flag in the window of his truck, or boisterous homophobes who hold up hate-laden signs at any event where they can get attention, as long as they aren't actively out hurting other people, they aren't violating anyone else's rights, so they aren't committing a crime, at least from the legal standpoint. Since I'm a law-abiding, American citizen, I'll just have to deal with the fact that some people are ignorant/crazy/perverted/etc. If I don't want to deal with that, I can move to Germany or Australia where speech is more restricted on the subject.