Boku to Kanojo no Midara na Utage

Tatsuya, a college student, is in love with his classmate Saki. However, he has a special propensity for cuckolding, and one day Saki finds out about it. "...Are you interested, Takkun? I'm having sex with other people." - With those words, the two start swapping with a close senior couple, Marina and Masao. Furthermore, a man and a woman appear who join their secret relationship, and the lewd party becomes more exciting...!!
(Paradigm Bunko, translated)