People, it's not what it looks like. All it would take is to see things from Reina's side, which will be revealed in a later chapter. To be clear, she isn't completely innocent, but as you will see, neither is the MC.
The manga is starting to show Reina’s side of the story and I don’t think that anyone would try and claim that the MC was a perfect boyfriend.
I don’t think that we’ll be seeing him do something that amounts to cheating but he clearly invests more into his friendship with Ayaka than Reina is comfortable with. Would it be the same problem if Ayaka was a guy? Is it because of time/energy invested, jealousy, both?
I know that it’s kept ambiguous whether Ayaka is into him or not but even she wonders about whether she was the cause of the breakup (was it during that hot spring trip?).
Regardless, the MC never crossed any romantic boundaries with Ayaka or anyone else and remained completely faithful to Reina. So Reina’s insecurities were probably being amplified by her friends, including this glasses guy with ulterior motives who just wanted her for himself.
To be honest their relationship was really odd looking back at it. They go to different universities so they barely ever see each other, so their relationship probably progressed pretty slowly. Also, they never met each other’s friends or family even after a year of dating!?! That’s completely bizarre. A normal person would assume that you were being kept away for a reason after just a couple of months. That’s how you treat your side-piece, not your partner.