The only thing i can say is that... the timeline is not clear in the sequel. but here it's blurry, but the gist is (lets be real here it the most iprotant thing) Kuramoto in fact did not cheat on Reiji.
Reiji on the other hand... I hope he find happiness in his story.
Well actually he did. If you follow the sequel. They were in a serious long term relationship before reiji went overseas. It was while he was over seas that this story starts then he gets back for this chapter. They haven’t broken up yet. So yes. They were together and yes he did indeed cheat on him.
What i have learned from Prequel Sequel mangas is that you need to get all the information and event timeline in the correct order, for all the readers to understand the whole picture and narrative of the story.
Exp: A story of an abusive seme and abused uke, you all would hate the seme instantly. But we would be missing a huge chunk of the Why and Where.
A Prequel shows that the seme was being abused and taken advantage of by his boss and thus cause the event into happening.
In the end the Seme knew he was wrong and let the Uke go. They were sweet hearts when they were young but in the end, it was not meant to be.
Note: Points to whoever notice which manga series i'm refering too.(▰˘◡˘▰)
The Ending "Chef's Kiss"
PS: Yoshiteru is tall, he towers over the other students!! How tall was Zenitsu when he became an adult, damn!?