Symbolism, Imagery, Allegory
In the idea of the virgin, several themes of the novel collide. First, there's the sexual element. Although we know not all the sisters are virgins (Lux is quite promiscuous), we know that they've been sheltered from sex by their mother, who thinks that boys are out for only one thing. The girls dream of boys and makeup and beautiful dresses, but the tragedy is that they die before they can experience any of it (except for Lux's rooftop trysts, which were more like a nightmare). The sexual nature of the virgin image also comes up as part of a song Lux used to listen to called "Virgin Suicide" about a girl who gives up her virginity.
The second element is religious; when Eugenides titled his book, he was thinking of the Virgin Mary, the mother of Jesus known for her purity and her suffering, revered by Catholics and probably very important to the devout Mrs. Lisbon. Mrs. Lisbon's religion is what makes her so strict with her daughters, what keeps them innocent and inexperienced (again, the rebellious Lux being a huge exception). This conflict between the girls' emerging sexuality and their mother's repressive religious ideas is one reason that the girls lose hope of ever having a normal life.
When the paramedics pull Cecilia out of the bathtub, they see that she's holding a printed card with a picture of the Virgin Mary and these words:
The Virgin Mary has been appearing in our city, bringing her message of peace to a crumbling world. As in Lourdes and Fatima, Our Lady has granted her presence to people just like you. For information call 555-MARY. (1.18)
Mr. Lisbon calls it "crap" and Mrs. Lisbon crumples up the card. We never learn what the image means to Cecilia, but to the reader, it's an image of a pagan virgin sacrifice. The narrator creates a living tableau out of the emergency scene of Cecilia's suicide attempt:
[…] the two slaves offering the victim to the altar (lifting the stretcher into the truck), the priestess brandishing the torch ([Mrs. Lisbon] waving the flannel nightgown), and the drugged virgin rising up on her elbows, with an otherworldly smile on her pale lips. (1.5)
Are we meant to think that Mrs. Lisbon has emotionally sacrificed Cecilia on the altar of her rigid religious beliefs? That Cecilia, a spiritual, ethereal girl, has offered up her hopeless youth as a sacrifice?
The Virgin Mary cards show up again a year later, just before the time that the other girls commit suicide:
Mr. Hutch found one tucked into the windshield wiper of his car [. . .]. Mrs. Hessen found one pierced among her rose bushes. Joey Thompson heard an unfamiliar whirring in his bicycle tires one day, and looked down to see a Virgin picture taped between the spokes. Finally, Tim Winer found a picture stuck into the grout of his study windows, facing in at him. (4.129-30)
The cards are a signal; everyone knows they're coming from the Lisbon girls who are trapped in their house, because they know that Cecilia held one of the cards when she slit her wrists. The Virgin cards mix sexual and religious metaphors just like the bra hanging on a crucifix in the girls' bedroom. It has a lot to say about how the girls are trying to understand their emerging sexuality while dealing with their mother's insistence on purity and religious morality.