Saikouchiku Monogatari - Uwaki Shita Kanojo o Yurushitara

The protagonist discovers that his cosplayer girlfriend has been cheating on him. The protagonist gets very depressed, but chooses to rebuild (start over) with his beloved girlfriend. While suffering from flashbacks of the affair, he somehow manages to regain his happy days. One day, however, an erotic video begins to be sold on the Internet and becomes a hot topic in some circles. It was a video of his unfaithful girlfriend being fucked.