The setting is a "delivery health" (dispatch brothel) service where all the girls are from "Isekai" (other worlds), and it would seem that semen is the only source for replenishing their mana in this world.
Aya Aono is my classmate, but we've never really spoken... until that day I caught her masturbating after school! Embarrassed as she was, she couldn't take her gaze off the growing bulge in my pants...