Kingin☆Heart☆Succubus!

Partly against his own will, Ataru has been specially chosen for an unimaginably important task: aiding three promising succubus achieve demonic transcendence. To do this he must rigorously assess the sexual performance of his charges, and in the process temper their strengths while correcting weaknesses. The reward for success is beyond anything his mortal mind could ever comprehend, yet the the punishment for failure is equally as incomprehensible.

[From an old review by Nargrakhan]