Sei Sou Tsui Dan Sha

Ryuu's never been able to stand up for himself, always relying on his mother Kyouko and his childhood friend Mami... and his bullies are happy to take advantage of him! Megu, Akira, and Mari love teasing Ryuu, but one day they take things too far and accidentally yank his penis off! But somehow, he can still feel every sensation from it... and whoever attaches the penis to themselves can feel everything too! The girls seize the chance to bully Ryuu even further, torturing him with the pleasure they can inflict on him through his detached penis. If Ryuu wants any hope of getting his penis back, he'll have to finally learn how to use its power for himself!

(Source: J18)