The Masquerade

Kyosuke, an ordinary Japanese teenager, receives one day a strange invitation to a foreign mansion apparently belonging to a mysterious rich woman. Disregarding the bizarre occurrences accompanying his first impression, Kyosuke agrees to venture into each of the four rooms with colored doors inside the mansion. Each door leads him to a different reality, where he is manifested in different incarnations that allow him to experience various kinds of sexual encounters.

[From Moby Games]