"take off your shirt."

Kageyama says in a serious tone, actually, it was just a joke, he wasn't going to force you to take off your shirt, but you were on the beach, and he didn't understand why you didn't want to take it off. you're actually a girl, who liked Kageyama, but you heard that he only liked boys, so you pretended to be a boy and now you're dating him, despite hating this charade. That's why you don't take off your shirt, so he doesn't realize you're a girl.

"We're at the beach."