Josou Kaikyou

Tajima Yukito needs to collect plant specimens for his research. Asking help from Hibiki. He comes to the island where Hibiki was born, but suddenly a storm causes their ship to crash. They awaken on an unknown island. They find two of Hibiki's friends who had gone missing, Fujikiri Nao and Munakata Meguru. Nao and Meguru say that they are trapped at secret location by the power of God. Possibly because Nao and Meguru did a ritual halfway. It's made it impossible for them to leave without finishing the ritual. According to old tradition in order to finish the ritual Yukito must mate with one of 3 traps, but Yukito is too embarrassed to do it. Now he must do it or they can't leave the island.

[Edited from Nounai Kanojo Website]