EDIT: I agree though that the parental response that you described is extremely unrealistic if that’s how it was portrayed. I haven’t seen the movie and never will.
It makes sense in context. The two are good friends well before it gets sexual. The dad knows and likes the older guy well enough to invite him to their house for the summer so it’s not like he’s some rando old dude. And there’s no “preying,” it’s entirely consensual.
Aren’t sexual predators often trusted family members or friends? And aren’t we taught that minors cannot give consent?