Those who read and write chanfic understand that chan does not advocate the activities presented in the text any more than rape erotica advocates rape. I think all parties agree that the exploitation of real children to create erotic material is a very bad thing. But just as the concocting of fake blood to use in a stage play does not create, in another person, the urge to taste or spill real blood, the imagining of an underage individual in a sexual situation does not make one a sick fuck who desires to hurt children.
And yet we have very real reason to be afraid to voice this. Australian pornography laws do not distinguish between erotic material that was created using a real underage individual and one that involves imaginary ones. American pornography laws have to date supported the precedent that prosecutable child pornography must involve a real underage person, but this does not mean that one cannot still be arrested and vilified in the court of public opinion, before a legal court rules in one's favor (one hopes).
So we go mum when it comes time to defend chan erotica, knowing if we don't say something that resounds right and true and (most importantly) benign, we're joining those who get the "sick fuck" brand. But our reasons for reading it and finding it erotic and knowing that we have no association between that and any urge to practice it in real life can be as certain as those arguments and statements that say, "Of course I don't want to be raped for real, just because I fantasize about rape," which most adults will nod at and recognize as true, even if they can't voice why.
Most chan does not appear to be about destroying the trust between an adult and an underage individual. Or, to clarify, at least it does not appear to be about the reader's pleasure in watching that trust destroyed. Often chan is about taking away all adult baggage of the sexual act and reducing it to solely a physical pleasure, guilt-free as tasting food when one is hungry or coming in to warmth when one is cold. This can apply whether one imagines oneself in the role of the younger or the older person.
Chan can be about giving up control. One can imagine oneself powerless to resist the other individual, who imposes the forbidden pleasure upon one. This too can apply to either role--the younger finding him/herself taken advantage of by the physical or authoritative prowess of the older, or the older taken off-guard by a willful (and perhaps villainous) younger individual. This is an argument often used to explore the appeal of non-consensual fantasies in general.
Chan can be about removing taboos, where bodies are made to feel pleasure and there is an attitude of "nothing wrong with that." Chan might be about maximizing taboos, where the pleasure is greater and more thrilling the "dirtier" the act is.
Chan can be unintentional. I think this applies especially to fanfic. We might identify with a character who happens to be underage, and want to imagine how we'd have reacted in the character's situation, and we impose our own adult feelings on that character when we enter that character's viewpoint, all unwittingly. So we imagine, perhaps, a sexual twist on the situation, and are not propelled by the character's underage-ness as such. This is where all those "Let's pretend they're all of age and this is an AU" excuses happen.
Chan where all characters involved are underage can fit into one of the above categories, or it can be a longing to return to one's time of total inexperience, to see all the characters on equal ground, when all discoveries are accidental, not to be blamed on any one character lording power over the other.
Once I would have assumed it was enough to use the "it's not happening in real life and doesn't represent a desire to do it in real life" reasoning and stop there. But courts around the world have demonstrated that that isn't sufficient argument. So, yeah, though I've shied away from these controversies in the past, I thought it was time for me to bring some discussion to the table.
The final point that I feel I need to add, for the record: I think that if you willfully go looking for pornography that involves actual underage people, it is no longer an issue of what you felt or intended when you looked at it--it introduces an implication that you approve of that kind of exploitation, and in the face of that heinousness, that is where I would say these arguments no longer provide any sort of defense.