I'm kind of tired of these discussions but I'm a compulsive dumbass and I can't help but want to say something.
1) As a general rule it's always important to ask yourself, "do I think this thing is immoral and reprehensible because it is causing harm to other people, or because I see it as gross or distasteful?" It's not always an easy question but it's an incredibly important one because feeling like they're on the side of morality can make people very bold, and that can have terrible consequences when conviction runs counter to human well-being. At the same time, letting terrible acts go unchallenged is also very bad. In short, it's something to put thought into, and knee-jerk reactions can be worse than unhelpful.
2) Real life child porn is obviously and inarguably reprehensible because the damage it does to its real-life subjects cannot be outweighed by any other consideration. The issue of depicting fictional underage people as objects of desire in media is open to discussion at all only because it doesn't require the harming of real children for its creation.
3) For an excellent discussion of why it can still be bad to depict fictional underage or child-coded characters as objects of desire even when no children are directly being harmed, see this excellent video. It make sense for this to be the subject of controversy.
4) Of particular concern in this case is the issue that, media that sexually depicts fictional children or child-coded characters can cause people to become less negative towards pedophilia by obscuring the reasons why it is so harmful. I think that, for the most part, when stories slap on, "this person is actually of age actually," it is essentially just self-justifying, and trying to deflect moral concerns, and does little to no real moral work; in fact, it can contribute to the problems I've mentioned by reducing the issue to a question of numbers, which hides the concerns of mental/emotional development, knowledge, and power dynamics that ground the inability of children to validly consent. Consequently, I think the more the story credibly addresses the things that make pedophilia harmful and separates the scenario it depicts from the scenario of an adult raping or grooming a child rather than sweeping them under the rug, the less harmful it can becomes.
As far as my opinion on this particular manga, I'm aligned with UnappropriatedFox and Nevri's points in a lot of ways. Honoka is introduced as an adult, and when it is made clear she looks like a child, the author wrote her with a developed and insightful set of concerns that an adult who people thinks looks like a child would have. She has her ID prepared because she knows how this goes, she's anxious about another partner leaving her because of how she looks because she has experience with that happening, and she casually mentions working from home. Her situation is addressed remarkably thoughtfully in the space it has in this porn manga, the author going the extra mile to not just nominally call her an adult but to show that she is, in fact, an adult with a fully developed brain, independence, an understanding of sexuality, and the ability to give valid consent. The manga even ends with a (frankly pretty cute) mature discussion of their relationship issues and needs that develops organically into them going out. Within the story's world, she is actually treated as an adult: it's not merely nominal.
Of course, that leaves the question of why she has to look like a kid in the first place. Yes, given Wakadori Nikomi's other work this is probably a case of author appeal and it's possible that the character was ultimately created this way because the author is attracted to kids, but I think any potential issue that might come from depicting a child-like body as an object of desire is strongly outweighed by the real good that this story does. By depicting Honoka as an adult with care and, critically, in a way that feels human and true to life (through those little details I mentioned), and the MC as someone able to overcome her reservations and treat her appropriately as an adult sexual partner and give love to her body, the author is able to actually represent an aspect of the human experience. Yes, it's porn, but it's porn that says, "it's okay to look like this. People can still love and desire you," and even if that's incidental it touches on people's real-world experiences and insecurities. As a trans person I know how valuable and healing it can be to have media that depicts you and says, yeah, you're allowed to be sexual, and your body is valid and desirable.
So yes. This isn't a, "she's actually 7000 years old!" scenario. It's not distorting people's thinking surrounding pedophilia. If anything, it sharpens that lens by showing that it's the mental development and power-related concerns that make pedophilia harmful, not the appearance of the person's body.
