It's about context and presentation. In most series someone going on a killing spree or committing mass murder or genocide is always presented as an immoral act, and we are always aware of that. The characters may feel like they are doing nothing wrong or justify it or even enjoy it but either it will be presented in a way that the readers will realize that this is just something the characters are doing and not something being shown to be morally correct, or at least in the story there will enough context to justify whatever act of violence characters commit.
Here in the manga (the sexualisation of a child) is not being portrayed in a way that makes that distinction. The daughter suggested a guy should touch her breasts (in a clearly sexual way) in front her mother and the mother's response is don't let it happen so easily? And the guy is clearly fantasizing about her daughter and she says don't get carried away and marriage is still on the table? And that is funny to you? I haven't read a lot of series where people kill each other for jokes but from what I've seen there's usually a punchline or satire and commentary or it happens in a ridiculous way, the killing someone part is not the joke. Here the sexualisation is being presented not as a joke but just a part of the usual hijinks. I honestly would not care a bit about this issue if this was straight up hentai because I have context for what they are going for, here it just makes me question the author's morality. It is kind of distasteful, they could have made the daughter older (honestly personality wise she seems older) or presented her in a cutesy (and non sexual) way.
Also in real life a kid killing someone is possible but it is so rare and far fetched in people's mind that no one will have trouble separating fact and fiction. But sexual exploitation of children, grooming, child marriages etc is a problem that is too real and may hit too close to home. It could be happening anywhere, it could have happen to someone you know, so it's a more touchy subject. Think about Fire force, a series about people combusting into flames spontaneously and other fire based disasters and it was airing with no problem, but while it was airing the Kyoani fire happened and they pulled it off air (don't remember how long but one or more weeks) and made changes because those things became too real in people's mind.
My main point is not that you are wrong but that people may have many logical reasons to draw the line here and have problems with this series.