Just coz it's fiction doesn't mean it's impossible irl. I'm not sure what do you base your morality on but any content depicting this type of situation whether fiction or not does not take away the fact that it could influence another person's decision and mindset. It's not uncommon for people nowadays to turn their "fantasize" to their realities. And these crime-related genres are very common in Japan, whether it be shota, loli, and more.
Do we just stop supporting these kinds of contents when they already cause harm to people? Never doing something to prevent it? In the first place, this kind of age gap is creepy even more so if genders are reversed. Because the other party is a minor who is less capable to think the same as a mature adult.
I completely agree with your first point. However, this is generalizing people and making the assumption that (for example) just because 5 out of 10 people act on the content they read, it means the other 5 also must do the same. Also, The age gap is creepy no matter the gender.. That's neither here nor there though.
I'm sure content like this absolutely could and (in some other cases) has influenced people into abusing children in real life. I am of the opinion that people who read this content and are then influenced to act on it in real life are mentally unwell. I do think you should consider though that just because someone reads this, it does not mean they also want to do it irl. As an example, if you play an fps game or watch a slasher film, most people will obviously not automatically go stage a shooting or murder a bunch of people. It's the same with a lot of people who consume lolicon or shotacon content.
I get it if you don't understand where I'm coming from, and I don't necessarily expect you to. Lolicon and shotacon is considered problematic media for a reason lol
I do hope this helps to explain a bit though!
I understand that it's fictional, and I'm not arguing that everyone who consumes this content will act on it in real life. My point is that fiction doesn't exist in complete isolation from reality. The things depicted in fiction can and do happen in real life, and I don't think the possibility of media influencing someone's perceptions or behavior should be dismissed simply because it's fictional.
The same principle applies to violent games and other media depicting harmful acts. Most people won't imitate them, but that doesn't mean the possibility of influence is nonexistent or that the content itself is beyond criticism.
Ultimately, my concern isn't about condemning every reader, but about questioning whether content depicting or sexualizing the abuse of minors should be accepted without criticism. If there's even a possibility of harm, I believe it's worth taking seriously rather than dismissing the concern on the grounds that it's fiction.

Imagine if the gender is reversed....and it has a romance tag on it