Harumi-chan no Oita

Wafaru is a guy next door. One day he saw a little innocent looking girl in his room. Her name is Harumi. She asked Wafaru if she could stay with him. She didn't have anywhere to go. Because of her innocence she doesn't know anything about a man's body and even her body. Wafaru showed her his member and explained to her it was his pet. She loves playing with it so much without knowing what it really is. Wafaru cannot live with the lies he continues to tell her. In a fit of guilt he has way with her without revealing his secret.

(Source: AniDB)