before Se7en and The Sixth Sense, there was this film.

It's Mia, not Maria.

At what point did you want to smack her? It's not like "Satanists got the devil to rape you, now you're bearing the antichrist in your womb," is a readily apparent explanation for her predicament. It seems like once she read that book, she was more than willing to see her situation for what it was.

Also, I think you're ignoring the social context in which the story takes place. A pregnant, middle-class wife was not exactly in a position of power in the mid 60s. She's under enormous societal pressure to go along with her husband and be the perfect housewife, not to mention the influence of her strict Catholic upbringing. With that in mind, I think her vulnerability makes perfect sense for the character, and her rejection of her husband later in the film seems like that much more of a triumph. And it's no fair to compare her to Ripley. This is a mid 20th century domestic chamber drama, not a grandiose sci-fi action movie set in the distant (and presumably, far more gender-egalitarian) future.