Physical cheating almost always begins with emotional cheating.
In fact, looking back at the story after Chapter 109, you’ll find that the author very neatly uses a synchronized approach to start Shinobu-kun and Sakura’s journey into emotional cheating, with each of them forming a deeper bond with the important new characters of this section—Nakanishi-kun and Fumi.
Nakanishi-kun vs. Sakura: Because Shinobu-kun went to the celebration party and was not there to accompany her, Nakanishi-kun and Sakura spent the whole night looking for the lost necklace. Early the next morning, Nakanishi-kun took Sakura home, and from then on the two of them began communicating more often. In Sakura’s mind, she came to see Nakanishi-kun as a good person.
Shinobu-kun vs. Fumi: On the day of the celebration party, Shinobu-kun got drunk and slept at Fumi’s place that night. Several shots were very suggestive, and then it was morning. What happened during the night was never explained at all; we only saw Shinobu-kun lying bare-chested beside Fumi, who was wearing only underwear. From then on, Fumi became the main figure in Shinobu-kun’s wandering thoughts.
Chapter 109 is practically a mirror image of Chapter 129:
Chapter 109: Nakanishi-kun and Sakura searched outside all night, until dawn.
Chapter 129: Shinobu-kun and Fumi searched outside all night, until dawn.
Chapter 109: (The male protagonist is taken to the supporting female character’s room.) Shinobu-kun and Fumi spend the night on the same bed in a suggestive atmosphere, one of them not fully conscious and the other fully awake. The process in between is left unshown; we only see the beginning and the result. Since the one who was conscious was the passive supporting female character, there was no physical cheating, and Shinobu-kun chose to hide what happened.
Chapter 129: (The female protagonist is taken to the supporting male character’s room.) Sakura and Nakanishi-kun spend the night on the same bed in a suggestive atmosphere, one of them not fully conscious and the other fully awake. The process in between is left unshown; we only see the beginning and the result. Since the one who was conscious was the active supporting male character, physical cheating happened, and Sakura brought up breaking up, choosing to hide what happened.