I'm not particularly familiar with US law, but as far as I know, incest is prohibited in all its forms. In Europe and Asia, it's a little different. There is no single law governing this. Although technically, content containing it is classified as adult content. What adults do in their own homes is nobody's business but their own. If he goes around shouting that what he does is normal, but from a moral and ethical point of view it is not, then yes, that's another matter. The question is this: if all adult content is subject to censorship, then what the hell is the point of age restrictions (this applies specifically to “adult” content and not to the sale of alcohol, for example).