http://www.mangago.me/read-manga/study_hard/mf/v01/c005/44/
nono he did cheat
"of course it was a lie, he had hardcore sex with a girl."
Uke was lying about doing it with a producer.
Shuri-shuri is right - he was faking the story about being forced to have sex with the producer, but he did have hardcore sex with a girl instead. That's how he ended up with the marks on his body.
http://www.mangago.me/read-manga/study_hard/mf/v01/c005/44/
Oh well, he's a douche anyway.
Did he still cheat in the end?
Flaky characters like that are terrible. Urg.