Porco and Fio's own grandfather's treatment of her is definitely written into the story to show what figurative pigs and chauvinists they are. IMO chauvinism is the very strong central theme of Porco Rosso. I think they are very much meant to be seen as character flaws. Fio is not sexualized from the perspective of the audience, just from the perspective of the adult men in the story (Though I've only seen the film, never read a manga of it).