Shin Hoka no Otoko no Seieki de Harande mo Ii Desu ka...? 3 - Uwaki Shuki ni Tsuzurareta Kairaku no Tsumeato

The protagonist, Masato, and his girlfriend, Sayaka, were a couple with about a year of dating history. They’d met during a regional joint theater performance and had been inseparable ever since.
Though they attended different schools, they spent almost every day together at home, deepening their bond.

In the fall of their third year, Masato and Sayaka participated in their final joint theater performance as students.
But behind the scenes of rehearsals, Masato remained oblivious to the fact that Sayaka’s body was being claimed by pleasure elsewhere.

The record of betrayal—The Cheating Memoir.
Everything was written there…

[Translated from dlsite]