If he had stopped because he suddenly remembered he had a fiancée and felt guilty, I don't think it would have bothered me that he aborted their encounter. It was the stopping and cutting Rachel down afterward that bugged me.
Agreed. It's not the stopping that bothered me. It was one, him pulling that crap in the first place, and then two, basically blaming her for his actions afterwards. She wasn't coming on to him, he's the one who pounced on her. So to then give her shit was ridiculous. And he said nothing about his fiancée, only about Rachel and her crazy.
Maybe she thought he was annoyed with her because of their last interaction at the end of Wife? She called him a whore and they didn't part on the greatest of terms. They both seemed to have gotten over that confrontation fairly quickly, but it might have been awkward if that was the first time they had spoke since it happened. Also Adam insinuated that he didn't necessarily trust Rachel to do what was best for Adam over what was best for the show, so he might not trust her that picking Faith was really a win win for him and not just spin from Rachel. Rachel might have thought he'd be in the mood to spite her.
I think she actually just pulled the reverse-psychology maneuver, because she knows he doesn't like to follow the rules. And we know that too, he's said it time & again, and proven it as well (note his continuing with Grace's secret blow jobs. I think it's about more than just getting lucky). By telling him to cut Faith, she guarantees he's going to keep her around just to spite her, not for malicious reasons, but because rebelling is just his natural inclination.
See, I don't think of Rachel/Adam as a "change the other person" fairy tale story. It's not about trying to change who someone is, it's about finding someone to be your best self with, and I think there's something about those two that makes that happen. And I think this week really showcased that, in the way that they put their manipulations to use for the greater good (the greater good), and both went to bat hard for Faith. I think that on their own, they're kind of shitty people, but together they're less terrible. They both call each other out on their shit, hopefully leading to endgame of making him man up and be more responsible instead of just whining about how no one takes him seriously, and making her lighten up and embrace who she is without succumbing to her mother's manipulations. They're not going to become different people. They're just going to be (slightly) better versions of who they already are.