Konna no Zenzen Erokunai!

Kiki, a beautiful woman who always sports midriff-baring clothes, 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 since become a researcher of belly buttons, Kiki decides to pay him a visit. However, instead of defeating Nakaba by making him cum miserably, she ends up having him pleasure her belly button...!?

(Source: Kodansha, translated)