Hentai Game 
Isekai Harem Fantasia has utilized the isekai concept as the title has an unpopular male being transported to a fantasy world full of monsters alongside his three female classmates, the male slowly starting to earn his keep as he eventually has sex with all three women.
Shortly after being whisked away to this alternate world, the protagonist and his companions join a guild to start gathering money and fame, tasking players with clearing turn-based battles and reaping sex as the reward (or even sometimes during defeat).
