Based on the erotic manga by Jyoka.

Once there was this kid, a simple lad who just loved life. Let's call him A-kun. One day, while he was playing outside, he sees a spooky figure off in the distance - in the form of a giantess. He ran back indoors, fearing for his life. His parents did all they could to protect him from this evil spirit. Despite their best efforts, the she-devil still managed to get inside the house. I'm not gonna go into detail about what happened next, but let's just say he used to be a virgin.