Koibito Swapping ~Shin'yuu no Koibito o Ore wa Daki, Ore no Koibito wa Shin'yuu ni Dakareru~

The protagonists, Koutarou and the heroine, Rina, are lovers who started dating when they entered the school. Koutarou is close to his best friend Hajime and his girlfriend, Mayu, who are in the same class and they often hangout as a group. At the end of exams, the four went to relax in the city, after they went back together to one house and hung out. It would have been the usual until then, but maybe there was a sense of liberation at the end of exams, the dirty jokes they were telling started to go in an unexpected direction. "Do you wonder about how others do it?" Showing each others sex from that one sentence. The appearance of a friend who usually hangs out with us, that you would not normally see. Their secret act that they have shown. Its a new excitement feeling emotions such as shame, jealousy, and immorality all mixed.... The actions of four people who fell in love gradually escalate. [Translated from Getchu]