Summary: Rachel will never forgive Blaine for what he did to Kurt.
Disclaimer: Glee does not belong to me
Rachel Berry could not forgive Blaine for what he did to Kurt. There was lots of reasons for this, and many people would call her a hypocrite after her own cheating ways, but there was two big fundamental differences between the two situations. Firstly, she had only kissed Noah, whilst Blaine had slept with this Eli guy (no, Kurt did not know the name, but she had her own ways in finding things out). Secondly, and most important, Kurt was a very different boy than Finn. Kurt had a lot of baggage, a part of witch Blaine had provided before they even started dating, that made him extremely insecure in himself. He had a long history of not being good enough, of being too much, of being too gay. Rachel knew that when Sebastian had started flirting with Blaine, Kurt had felt that he had to grab Blaine with both hands so that he couldn't slip away. He had felt like Blaine had no interest in fighting for them. In general the last year had been tough on him; the election, Tony, NYADA, Blaine withdrawing from him and then the cheating. For Rachel it wasn't so much about the cheating itself, which was bad, of course, but more about how it made her best friend feel. Kurt had told her that Blaine had blamed him for not being there, and maybe there was something to that, but all Rachel could see was that Blaine had cheated to punish Kurt. Blaine knew how bad Kurt self esteem really was, and must have known how it would affect him. When Blaine said he hooked up with someone, Rachel knew the only words Kurt heard was “You aren't good enough”.
And Rachel Berry would never forgive Blaine for that.