To some people blaming Ayaka, are we reading the same story?
-Ayaka never hogged on Yuta when he factually started dating another girl and since highschool they had this pact of not becoming romantic because of the shit that just had happened to them. Ayaka made an effort to fix her self admitedly faulty personality (which is most likely the spark of whatever clash she has with the kouhai) and by her college days, she is mostly friendly with people, 100% sociable and approachable and has a healthy bromance with Yuta. She offered advice when solicited and even distanced herself from him enough to the point of telling him that he should be having his own input about how he interacts with his girlfriend in the first place. It is ONLY after this romantical crisis her pal had and after seeing him so crushed that she considered to seriously approach him in a romantic way on her own pace. And even then when they were in solitude she was reflecting the possibility of having been the cause of this outcome. Both the MC and her only found weird right then about this being a possible scenario because they had meet at best 2 times before.
-Reina aside, the glasses girl is the real source of conflict. Even the MC has noticed how weird it is that she known too much about him or his relationships with other people. It's clear even before they meet physically in the mixer that she has been observing him and making up her own conclusions. Give it a reread and you will notice how creepy it is the way she instantly appraches to click with him.
-But on the topic on cheating, they made a lot of emphasis on how Reina behaves around people and around Yuta and the real significance of the handholding to try to play down what she did regardless of the circumstances or her insecurities. She got to the point of doing with someone else what she herself stablished as special with the MC. It doesn't matter if she isn't engaging in other acts of the carnal nature or not. Matter of the fact, and how the MC presented it, you can imagine anything else, because trust (a central topic on this story) was already breached.
-The MC himself has been 100% transparent about everything regarding this conflict, even trusting Reina enough to tell her that Ayaka is a very important and significant friend to him and that she must meet his girlfriend no matter what because he thinks she is just as important. He laid the ground for that friendship to happen, there is 0 room for doubting or blame shifting. At the end of the day this is pure sophism to try and save face for self satisfaction. The MC could just tell Reina that she isn't a cheater and nothing would change regarding his decision. Why should he bend to someone that breached trust?
edit: aforgot to mention that every single time that Natsuki or Reina have appeared in panels I've felt emotionally distressed, at the very least this manga does the job of convery the MC's mental state very well at least for me.