13-Sai is a totally different manga. This one has been intensely creepy since the first chapter. Remember, she was gonna fuck this kid. Straight up. Until he said that he just wanted to experience having a mother.
This is, however, an interesting title. Because it's horror, and the horror is the actual confusion, fear, and social consequences of being preyed upon. Remember, the reason he's a loner at school is because rumors went around about the date he went on. Instead of that being something people were concerned for HIM about, they called him a weird loser and didn't wanna talk to him anymore.
We've seen it already multiple times, but look at this chapter: Saki is deliberately and intentionally taking advantage of Jun's lack of an example of a healthy relationship between mother and son to be super, super inappropriate. Washing his back while talking about how broad and manly it is, washing his arm, then washing up and up his leg until...
She's giving him the full Soapland experience, even playfully blowing bubbles on his face. THIS IS NOT HOW A MOTHER AND SON INTERACT. But he doesn't know that. He feels like it's wrong, but he can't fully reject the idea that she's doing anything other than what's normal for her. Which, actually, might be true! It's still totally bad, but it may be true!
The first chapter is about the male lead hiring an escort as a child and that escort being 100% ready to fuck him until he said he just wanted to know what it felt like to have a mother. It didn't turn dark, man. It always was.