Bullshit, it's not a prequel or a prequel. It's a continuation of the story. While Naoto was having fun with Ayumi, it turns out Suzu was alive the whole time - her body paralyzed and her soul in the ring Ayumi wears. He has to be careful to keep it a secret so that he can inherit more of his children, but then he returns Suzu to normal, and Naoto has to have children with both of them. The three of them get married, and that's the end of the fairy tale. The Japanese are screwed. In his first appearance in the series, he has also shown that the only thing he does is take Suzu's virginity, and he has knocked Suzu up and gave her a bellyful of baby food. Then we can consider this hentai work as an alternative version of events. Both get pregnant by Naoto and marry him, each having a child.