Everybody knows molesting a child is bad, everybody knows that in almost all cases dating a minor or having a large age-gap at younger ages can be problematic. As long as you know this, I don't see the problem with these kind of stories*, they're just wishful thinking - it's exactly because they can't work in real life that people who enjoys them like the whole idea : also, hentai or violent stories aside, those kind of stories tend to be filled with diabetes inducing sweetness.
I guess it's an unpopular opinion and most people will keep seeing those stories as a way to make paedophilia acceptable (since there are still people believing videogames makes you into a mass murdering psychopath) so, in the end, it will probably be censored.
It's complicated because you have to consider not just the effect on say adults who read it but also on children who get exposed to the more "acceptable" stuff on TV and such. And the thing is there are people who wanna make excuses for pedophilia and make it more acceptable. Those people exist.
Whenever this stuff comes up people try to use what they perceive as conventional wisdom, which is really just another way of saying something is your own biases and what you take for granted.
The existence of loli manga and loli fanservice may not turn someone into a pedophile, but it could say dull their reactions to how and in what contexts its presented and how it might affect others. An example of this would be people arguing there's nothing wrong with sexualizing child characters in an anime for children. Could the prevalence of loli fanservice and loli anime fed that perspective? It could have, but we don't know for sure. Just like we don't know for sure what negative affects loli fanservice in children's anime might affect them.
It's complicated and ultimately with the law you have to draw an arbitrary line of where you think the risk of allowing certain speech outweighs the risks of restricting it in a particular way.
A thing people also have to understand is that media doesn't always affect us in the ways its designed to, and it doesn't always affect us as much as we expect it too.
Fight Club is a condemnation of Durden's philosophy. He's the villain. At the end of the day, tho, a lot of people leave that film liking his ideas or thinking he's badass. That wasn't the intent but it ends up being the takeaway for a lot of people.
Nothing is straightforward.
I say this as someone who thinks obscenity laws are often a dangerous tool for government to have. I say this as someone who thinks loli porn manga and such shouldn't be illegal, but that its distribution should be regulated.
At the end of the day, there's a lot we don't know and a lot we don't have studies for, and no amount of "It's definitely bad" or "It's only bad for people who are already dangerous" is gonna change that.
There can be value in even the "bad stuff" too. Victims of abuse who use porn of that abuse as an outlet or a coping tool. I don't want drawn loli porn to be completely illegal because there are people who project themselves onto the lolis in that stuff and it helps them deal with the struggle of living day to day.
Some abusers use loli porn to groom their victims. Some use normal porn to groom their victims. Is loli porn more successful at grooming victims? As far as I know we have no evidence one way or the other. Do we ban because it might be? Do we not ban because we aren't sure? Everything is gonna be about weighing pros and cons and different people are gonna weigh that stuff differently.
tldr: Nobody knows as much about this stuff as they'd like to think they do, as so rather than using their assumptions one way or the other to argue your point, accept that maybe this won't be solved on a yuri manga message board and focus your time and energy more productively.
Like on how adorable Mimika is or how many good gay anime there are this season.