Umisen Yamasen

Main character hurries to leave home town after graduating from high school.
One day father priest sends him a telegram and asks to come back. Town has turned into an almost extinct village over this time. Father asks protagonist to get rid of the spirit of sea and mountains that drive away all the fish and attract beasts. But the way to awaken power and drive away the spirits is to have sex with them.

[From vndbreview]