Konna no Zenzen Erokunai!

Synopsis

Kiki, a beautiful woman who always wears a bare belly button, is troubled by the fact that she can't have sex with men. The reason for this is that Nakaba, a junior from her middle school days, has left her with no other way to derive sexual pleasure than through her belly button! In order to purge Nakaba who has made her body like this, Kiki decides to visit him, who has since become a belly button researcher. However, while she wants to defeat Nakaba by making him cum miserably, she ends up having him pleasure her belly button instead...!? A new door opens in this romantic comedy about unusual fetishes! (Source: Kodansha, translated)