I Became a Prisoner Trainer in the Dungeon

I was reincarnated into a fantasy world without any cheat abilities. After scraping by as a mercenary, I was captured by a commander in the Demon Lord’s army. “I hear you’re skilled in bed? Follow me to the training chambers.” To survive, I agreed to become a s*ave trainer who conditions human captives. If I fail, it means death? When I die, I go back in time—to the exact moment of my last clim*x inside someone.