That whole manga aged better in my mind, than it did in reality, but the last few chapters are not that bad. Though it's hard to stomach the needless drama. Today's protags would have confessed by that point. Weeks ago even. You notice, a lot has happened in the way such stories are written in the last 10 Years.
Welp. Of course MC is a BIG, BIG idiot, if he starts to date the new girl. By the old standarts, that is. Arisaka and Yuji are both idiots for not sealing the deal on that balcony. Sure they know each others, they are embaressed, but this is what happens, if two potenional lovers dance around each other for too long. Too high stakes, too afraid AND too proud to confess, because it might go wrong. Wrong time, badly executed, but do I also smell a hint of too much "too afraid to be emotionally destroyed, if turned down" in that scene? I believe, I do. The last thing happens, when you go over a certain point in that dance. Fun's over and things become awkward. Their relationship will spoil, if this continues.
At this point he couldn't even finish his confession anymore and Arisaka is an idiot for not realizing. He is an idiot for not going through with it, even though she started it with "Is there somebody else you like?" and it is clear, what she meant with that. It's also clear, what he wanted to say, but she felt hurt, because he didn't go through with it. And then she gave a really BAD reply. Not bad in the way she fraised it, but bringing the other girl back into this conversation at that point, even though she wanted just to end the conversation, is seriously BAD.
New girl made a good point for her case. Yuji has to make a split second decision, that might alter the outcome of his life. Is he willing to give up on his childhood sweetheart and ready to move on? Well, the good thing is, no matter what decision he makes, it won't be a mistake.