It is well-established in the law that First Amendment protections do not apply to obscene material--where "obscene" is not just "pornographic" but "sexually explicit in a really really gross way with no redeeming social value". There's a summary of the relevant law on the DOJ's Web site here:
I'm not saying this a good law, but anyone who has this kind of stuff should know that the "it's just ink on paper" argument cuts no ice with the judges.
I do not believe that these manga depictions of sex with minors have "serious literary, artistic, political, or scientific value". So, I do not believe that they constitute art and therefore it's OK for the government to intervene.
We're getting somewhere. So why do obscene stuff should, in your opinion, be controlled by the government? I don't mean why it's distasteful or unhealthy or something like that, but what makes you want to cross the line from "not socially accepted" to "men with guns ruin your life"?