Because she saw how nervous he was and wanted him to know that she won't be disappointed. Then, when she realizes how it actually affected him, she pulls back because she doesn't want to force him. Then when she leaves in the morning, she kisses him on the cheek and whispers into his ear "see you," saying to him that hey, I understand that this may have been too much but I want to see you again. Confirming through her act of affection that she's not disappointed in him.I mean, why would you say that to his face? Unless that reaction was what you wanted?
Like she's... fucking doing everything right here, these past two chapters are frustrating beyond belief. Azuha isn't messing with anyone's emotions. She's gentle, inviting, reassuring and even pulls back when she's sure she took it too far and making sure to communicate that she still wants to see him. This, WHILE she knows that he was talking to someone else and threw that woman out of his room with a lie, clearly not confident enough to even say that the two of them were in a relationship. Nor confident enough to tell Azuha that someone else was there.
I don't think that's fair to him. It's less "oh no, she's impure," it's "oh no, I'm not going to be anything she remembers." Her words that were meant to re-assure him that she isn't going to dislike having sex with him instead made him retreat into his feelings of inadequacy, which she fucking noticed.Oh God he is a cringe purity worshiping loser.
The frustration in this manga isn't that he's a purity obsessed weirdo. No, they wouldn't have had this thing if he was like that. The frustration is that he is deeply, traumatically insecure and he seems to refuse to take Azuha's actions as the gentle and re-assuring things they are, instead being mysterious and inscrutable. It was cute and honestly kind of interesting, because we had a both unlimited but limited perspective; only ever hearing his thoughts in times of anxiety and insecurity. He can talk to this beautiful and affectionate woman about their shared hobbies for hours, but when the prospect of her liking to be around him and wanting to spend more time with him pop up, that's when we're teleported into his horrified and disbelieving head.
But now there's some OTHER girl and its annoying. Azuha doesn't deserve this. She's been so patient and careful and clear in her non-verbal communication that this relationship being denied is deeply frustrating, especially because we've never seen any acknowledgement from him about just how much she's trying to say she likes him and wants to be around him. Cause he can't, yes, cause he doesn't believe it, but god DAMN it man what does she have to DO dude