OK.Spoiler Alert.(づ ̄ ³ ̄)づ
Masato was one of Kousei's best friends before. And he keep telling Kousei his firt BF would leave him as that guy used to be straight.Meanwhile Masato found Kousei's BF was a little suspicious that Masato and Kousei might have affairs ,so Masato tried to pretend more like the way they did have affairs especially in front of Kousei's BF...(● ̄(エ) ̄●)
Kousei used to think Masato was right and was really thankful about what Masato did as a friend even after he broke up with that poor guy...But he finally heard from their common friend, which is also Asahi's friend, Naru that Masato was always enjoying testing and ruining this kind of “straight-gay ”relationships after Masato got hurt by his "straight" ex-boyfriend Kazuomi...( ̄∇ ̄")
Yup, sounds like Masato wasn't actually a jerk to Kousei at all. It actually sounds more like something an overzealous friend would do, now. So now I'm really hoping that Masato isn't going to be put in an even more painful position, at all. Btw, did you mean to reply to me with this or the other poster?

Sorry, nope. The uke is an ass, he hurt people who have nothing to do with his own problems, yes. Here's the thing, though, bringing the ex into the equation AFTER the uke starts regretting what he's done and he's started to act more like the person he originally was BEFORE the trauma that lead to those actions (something else I have a problem with, because, once again, he's putting more effort into a relationship than the other party is - which is a clichéd trope - but that's another story) signals to me that the author believes what happened to the uke was acceptable while him doing similar things to other people, because of what happened to him, is not. Not only is that contradictory 'logic', it's also very victim blamey. I, in no way, think that the ex has to forgive the uke. But I think the timing is very shitty, given what I've said, already. In my opinion, if the ex showed up before he showed any remorse and witnessed the uke changing, that would have been more acceptable yet not put any more of a burden on the ex to forgive the uke.
Wow, I see a lot of people love trashy logic and abusers and dickheaded semes. Downvote only if you agree!