Based on the CG collection by Aomizuan (青水庵). 
 
The main character, Seiichi, enjoys traveling to remote islands and is a teacher by profession. He is a teacher, and is a man of immense sexual prowess, but he is not satisfied with his daily life. One day, he receives a request from the chief of an island he knows. He is transferred to a remote island called Koumi-jima, also known as Childbirth Island. 
 
The "favor" was... 
The women of this island have evolved a unique constitution, with only one week between implantation and birth, due to the island's custom of specializing in the role of females, and have continued to thrive in their own unique way for hundreds of years. It seemed as if the island would continue to prosper at this rate, but because the islanders have been mixing only close genes among themselves, it has come to the point where only genes from outside the island can be used to produce the newborns. 
 
The island chief's request was for Seiichi, an immensely fertile man with an outside gene: "I want you to impregnate all the girls on the island" 
That's what it was! 
 
The island's custom of specializing in the role of the female, the women want to satisfy the instincts of the men, who from the bottom of their hearts have the idea of male superiority.
What will happen to the women who feel the supreme pleasure in pleasing men and giving birth to children when Seiichi appears on the scene??

Koumi-jima: Shuu 7 de Umeru Mesu-tachi