I think we can make a meaningful distinction between not carrying smut and engaging in viewpoint discrimination. When it comes to government censorship,, First Amendment doctrine easily distinguishes between content neutrality and viewpoint neutrality. Content neutrality is the broader concept but applies in less circumstances than viewpoint neutrality, which pretty much applies anywhere. So there's plenty of venues where the government can ban nudity and pornography and such, but can't ban opposing viewpoints from being expressed.