HMMM? What are you even saying? Once you cross a line can you be trusted not to go further? Once you break trust, can you really be trusted to never break it again? And if there are people out there who are not willing to trust you, can you really blame them?Throughout the story Ayaka was Yuuta's crutch to solve problems and they were insanely close. I can definitely understand someone who was okay with it at first but then started to catch doubts. Reina should have just bit the bullet and asked what was troubling him but she didn't and that was where she messed up because at some point seeing the message from Ayaka made her overthink and doubt and reach a breaking point. If Yuuta was having troubles he honestly should have gone to his girlfriend first why did he go to Ayaka in the first place? As you said yourself if it's not a big deal why not ask Reina?
Yes her thoughts which came from a place of self doubt and wondering if she can also understand Yuuta in how he sees himself with ayaka. Even at the start she didn't want to go through with it but as the chapter progressed her self doubts began to win and she made a bad decision which seems didn't go much farther than handholding before it ended anyway. Here is the thing reina doesn't see any body contact with Yuuta but we as the readers did. Ayaka was clearly insanely close to him that a lot of people could misunderstand its a point that's been said multiple times throughout the story. So what if they don't show any kind of "lovers" contact? Contact is contact and for some people any kind of contact can be too much considering some people even think handholding is too much.
And throwing a hyperbole of reina moving on to actually cheating instead of having a moment of self doubt is laughable.
And this is coming from someone who experienced both sides of this kind of conflict.
It's ALL Reina's fault. EVERYTHING. She was dumb, easily manipulated by those around her, and she kept all her doubts to herself. And yeah, she made a mistake, but you see SHE MADE A MISTAKE. Everything that happened was because of her actions and inactions. The ball was in her court the entire time, and it was her decisions that caused everything to fail. I don't care about the WHY, I'm only stating the facts. SHE had doubts, SHE kept quiet, SHE tried to cling to a male friend to see if she could emulate what MC and Ayaka had, SHE allowed MC and Ayaka to continue their relationship as normal, SHE listened to her stupid toxic friend, and SHE chose to stay quiet even after being found out of what she did, simply returning the extra key MC had given her.