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.
"Let's make a 'I'll show you mine if you show me yours' pact. It'll be a secret just between the two of us." A low-energy, laidback classmate who's out of your league makes you a secret offer. Enjoy a thrilling, naughty relationship based on "showing" alone together with her. In the stairwell, the empty classroom, the library...
Certain family circumstances have lead to you living in a women's dorm. The girls there want you to have a memorably nice time... get ready to get spoiled!
"It looks like my husband is coming home pretty late. Want to go get dinner?" That's the line that started your secret relationship. And eventually, a simple ear cleaning turns into something more...