Most people have not yet read Nymphet, since we haven't even published it yet, so I would like to clarify an important point: Nymphet is a story about a mischievous young girl who tries to sexually entrap her teacher. The important context here is that the girl's advances on her teacher are never reciprocated by him; her teacher is horrified by her actions, and his romantic interest is in fact another adult teacher. The comedy arises out of this young girl saying and doing improper things (much like Crayon Shin-chan, which is currently being aired on Cartoon Network) and seeing her teacher squirm with discomfort and shock while he struggles to keep his composure, at the same time trying not to make a fool of himself in front of the woman he loves.
What's the point he is making there? He's saying that the situation is one where a little girl sexually teases an adult teacher, who isn't interested, no sex takes place, and (by implication) that's better than the reverse situation, of an adult molesting an unconsenting child. His point: the instigator makes a difference. A sexually aware child being the instigator with a responsible adult who acts responsibly (so that nothing actually happens), is better than a sexually unaware child being exploited, with an irresponsible, criminal adult as instigator.
In real life, that comparison seems obviously correct. Rape is really bad, right? But as for fiction, I think he may have overlooked something critical about North American culture which differentiates it from the context where, as DeAngelis goes on to say, Kodomo no Jikan "is not considered pornographic by any means, and Japanese would be shocked to hear this sort of accusation about what they consider to be a mainstream property." The missing insight is that the "mischievous young girl" scenario directly conflicts with the script for adult/child sexual interaction baked into almost all North American thought on the subject. We know how the world is supposed to be, we know how the world must be, Kodomo no Jikan doesn't fit into that world, and it's unimaginably offensive to many North Americans because it's literally unimaginable to many North Americans.
As North Americans imagine it, children are not sexual beings. Sexual contact (even if only potential) between adults and children has to be framed as an adult pervert exploiting an innocent child victim. That's just the way it is and the way you're required to think about it. You are not allowed to frame it in other terms. If you do, no matter how strong the facts may be on your side, it's you that is wrong, not the social construction. Those statements are made explicit in sources like R. v. Sharpe with its talk of "cognitive distortions": there are some thoughts that are the wrong thoughts to think, and a child as sexual instigator instead of sexual victim - indeed, a child as anything but an object - is one of those wrong thoughts. Good people aren't allowed to think that.
It's also evident in R. v. Beattie, where stories of adult/child sex were ruled to be especially repugnant because they were not about rape. Figure that one out. Stories about adults raping children, no matter how upsetting, at least fit into the social construction. Adults are always the instigators, and children are objects rather than active participants. But just let a child not be an object - let them be a willing participant, an aware participant, or worst of all, the instigator, and everything falls apart. The nicely constructed understandable world doesn't work anymore if it becomes necessary to think about children having sexual awareness and sexual desires of their own, and any ensuing discomfort must be the fault of the person who mentioned the idea. Children must be objects! Hang the man high who says otherwise!
So along comes a manga with a story that demands to be interpreted in terms of a child who is not an object. She's a sexual instigator, with an adult as arguable victim, and it doesn't matter if no actual sex acts ever occur and it never gets explicit or pornographic under objective definitions. The story concept itself is the real problem. Remember how offensive Lolita was back in the 1950s? Teenager as instigator was only one of several possible interpretations for most of that book, arguably presented as Humbert Humbert's distorted view of his own abusive activity, and even so, it really freaked people out. It's pretty clear from the last 50 years of history that we haven't learned much from Lolita, and now it should be predictable that Kodomo no Jikan - where the child is even more clearly sexually aware and sexually aggressive - freaks people out on the same level. This manga is based on a premise of children as real people instead of objects, and that is Not Allowed. That's the real reason it's unacceptable for the North American market. This is not about the sexual content itself, which (with some exceptions, as described in DeAngelis's second statement) seems to be tame in comparison to other things we tolerate.
A whole lot of fuses are going to be blown in a whole lot of people's brains as we're forced to confront cases like the 2004 Pittsburgh self-porn case, in which a teenage girl was charged with a bunch of sexual abuse and child pornography crimes for distributing naked pictures of herself. All by herself, that is - no adult perpetrator to blame for it. My comments on that article got a lot of linkage recently, even though it's a few years old; similar cases continue to happen and will continue to happen. It's going to be a problem as long as we try to maintain the fiction that human beings are objects until they're 18. It's also going to be a problem as we're forced to confront other cultures that don't share the North American cognitive distortions about children. When will we come to our senses?