Kanojo wo Netorase Fuuzoku ni Nante Tsurete-kun ja Nakatta

At first, it was just a light interest.. Yuu, who enjoys NTR, asks his girlfriend, Hayu, to go with him to a cuckold brothel on his birthday. She is reluctant to do so, but she has no choice but to go along with her boyfriend's hobby just once. --But she doesn't know that this is the beginning of her mistake. (Source: Fanza, translated)