He didn't take Shirosaki playing the "I'm an innocent little girl who doesn't want anyone else to be burdened" stereotype into account.
She's definitely going to be mad at him (I mean duh), the question is WHY will she be mad at him.
@imahuman She will be mad because this seems to be, in her mind, kinda like him cheating on her. Imagine if your boyfriend/girlfriend (which you may or may not have forced into going out with you) was told to kiss a person in the cheek right in front of you, then the they double down and say: "know what? cheeks are for grade schoolers, I am going to french kiss that bitch right here, right now".