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Frank was confronted with the knowledge that Erika liked him and that he could abuse that for his better negotiations with the werewolves, but to him this felt rather wrong, and he would very much not do such a thing. Playing with the feelings of a girl felt wrong to him…
As someone that grew without even making any girl attracted to him until recently, he still had within him the thing about not feeling right about playing with girls around. First of all, he never saw girls as toys to begin with, so he can't really do something as easily as they're telling him that it is.
He was after all a humble boy, even after all his developments, he still wasn't an edgy brat, so he was trying to balance his "dark side" with also his human side, due to this, playing with someone's feelings, especially the honest feelings of love of a young girl, felt horribly wrong to him.
Check "The Protagnist's Sister Is Actually the Strongest"!