Admittedly, what he did was wrong and he should have been honest with Michelle either way- but especially since he continued contact with Natalie. Also, he should have notified Michelle if he perceived them as officially "separated" and not working on things. But- I still think what Michelle did was worse.
Michelle emotionally cheated for (what seemed like, not sure of the actual chronology) months before actually cheating. I understand she wasn't happy happy- but she had thought about it, encouraged it, almost premeditated it. It was a mistake, but it was a choice that she made by default of not taking initiative to end things directly.
Brett made an impulsive decision, and is now floundering and handled it stupidly and immorally (in my opinion), however- he's trying to figure out his life. I think she pulled the high horse on him with that out burst.
I do feel bad for her, but I don't think what he did was worse. It was worse in that she was making an effort at the time he cheated, but, too me- her emotionally cheating and playing into it for months outweighed his impulsiveness. (Neither of them handled the afterwards part very well, but I guess it's not easy to handle.