Geten no Hikari wa Subete Hoshi

Main character with short-term memory problems is invited to a detective office. Woman Machiko, who seems to be director here, listens to his problems and goes to another room. She calls protagonist to proceed there, and there sexually assaults him. She explains that he was possessed with evil spirit and needed ejaculation to get rid of it. Now they need to restore memory about where that evil spirit could be contracted.

[From vndbreview]