Main problem I find is Reina... Sure, MC being dense and all caused a problem but remember...
In the earlier part of their dating he already talked about Ayaka and Reina said nothing of it AND as well as once around the middle, MC asked Reina if she was fine with him talking to Ayaka and she said it was fine.
Well, to be fair, she really didn't have doubts back then and she was totally fine with it, but because of that fact, MC now holds no reason to think that Ayaka would be a problem for their relationship since Reina herself said she's fine with it.
I mean, I understand that it would be hard for Reina to bring it up specially since she's the one that approved of it in the first place (or atleast I think I would have a hard time bringing it up if I were in her shoes), but because she never spoke up about it when she started to feel uncomfortable with MC's and Ayaka's relationship later on, I don't think she has any grounds to defend herself against that cheating she did regardless of "why she did it".
Another point with this is Natsuki's being a huge antagonist in this story since she's saying that MC should have set boundaries and made things clear BUT MC did try (although not a straight up talk with Reina) to set boundaries since he thinks the relationship he have with Ayaka would be bad for the relationship AND it was Reina who actually set the boundaries and established that their current relationship was fine with her so I really really hate that Natsuki is acting like Reina has done nothing wrong and MC should have clarified things with her.