This is an NTR story that began when Rin boasted to his best friend that "I have sex with my girlfriend five days a week." His best friend is a psychopath, and he thought that the remaining two days a week were a waste, and that he should increase his friend's sensitivity by using those two days a week. Rin was confused by his best friend's insane behavior, but he accepted it, and as his portio was developed, he became addicted to the vaginal orgasms he had never experienced before...