I mean he's already wary of her after quite a few "innocuous jokes" like this.Ngl i get they are creepy etc but low-key feel like sometimes author just makes things that would be perfectly normal be "creepy" like when the daughter coming onto his bed and messing about prior to her saying anything crazy he already said it sas scary...like bro she's a kid just messing about on the bed. It's only after she says she was the first that was It was weird
Even if she was only using some dirty humor to tease him, it would still be normal for him to feel uncomfortable.