Does she not realize what she's doing? She's threatening to hurt him the same way his ex-gf did, which, if she still didn't realize, badly traumatized him.
I hate it when mangaka's do this, give a character a trauma and then forget about it. He should be having a greater reaction to this than that, there are never any repercussions for the FMC when they act sly, almost every manga has this fmc that acts cute while threatening the MC and expects them to return their love. It frustrates me to no end.