I absolutely believe that sex with children should be socially unacceptable. However, I believe that fictional depictions of unacceptable acts are acceptable.

By outlawing depictions of one unacceptable act because it has crossed a threshold from "unacceptable" to "super-unacceptable" such that drawing it is illegal, you open up two slippery slopes:

1) Other crimes may be deemed "super-unacceptable", creating criminals out of otherwise law-abiding comic collectors.

2) Other mediums may be deemed off-limits. Could a written description of sex with children be illegal? Could a spoken description of sex with children be illegal?

In an extreme worst-case Orwellian scenario the law would fall down both slopes: any depiction of crime in any medium would be illegal.

Or, if I wanted to play the score-free-debater-points game the other way, I'd say "You don't get a free pass on incitations to murder or solicitations to ponzi schemes or offers to sell marijuana just because they're written. Why should 'it is just ink on a page' save child pornography when it saves none of these things?"

You don't get a free pass on a written note that says "let's go make kiddie porn" either. I'm not making the "just ink on a page" argument, which doesn't save the examples you gave or real child porn, so it shouldn't save explicit manga either. What should save manga is that it's fictional; no other crime was committed in the creation of the work.