Her not thinking that she did any wrong to Peter doesn't mean anything. My best friend was abused by her husband for years, and I wouldn't be surprised if he still doesn't think he did anything wrong to her.
You're making assumptions about what the wrong Peter did to her was, and I don't see those assumptions as being justified by the text. (though I'm not about to buy these issues for more detail.) Yes, it could be the sex itself. It could also be "I can't believe I fell for another jerk. Hey, wait, he does care about me," which would be stupid on her part, but not unlikely. Or it could be, "Damn, that's insulting, when I sleep with a guy, he always wants more. Oh, wait, he does want more." Neither possibility speaks well of her, but from what I've seen Peter doesn't come off well, either. If he can be a jerk, why shouldn't Michelle be an idiot?
I understand her initial anger, but at some point, she should have taken responsibility for her own actions and realized that if Peter was so drunk that he can't remember the night, she shouldn't treat him as though he had been sober all along. The impression I got about why she was upset was less about the sex itself and more that Peter was acting as though the sex was unimportant to him, which would be valid if he had been sober, less so if he literally can't remember it. We have no idea how or why they ended up in bed together, but as the more sober one, she should have been the one to stop it. Granted, she probably wasn't in any condition to be that rational, but she should still say to herself "wait a minute, he was more drunk than I was." So now we get into another way of looking at her actions. Maybe her anger at Peter is displaced anger at herself.