In The Secular Scripture Northrop Frye takes a look at romance in its broadest definition and observes that there is a lot of emphasis on the heroines' virginity.
One can, of course, understand an emphasis on virgniity in romance on social grounds. In the social conditions assumed, virginity is to a woman what honor is to a man, the symbol of the fact that she is not a slave. Behind all the "fate worse than death" situations that romance delights in, there runs the sense that a woman deprived of her virgniity, by any means except a marriage she has at least consented to, is, to put it vulgarly, in an impossible bargaining position. But the social reasons for the emphasis on virginity, however obvious, are still not enough for understanding the structure of romance. (73)
Deep within the stock convention of virgin-baiting is a vision of human integrity imprisoned in a world it is in but not of, often forced by weakness into all kinds of ruses and stratagems, yet always managing to avoid the one fate which really is worse than death, the annihilation of one's identity. [...] If we want an image [...] for this kind of integrity, there is an exquisite one in Sidney's Arcadia, where the heroine wears a diamond set in a black horn, with the motto attached "yet still myself." (86)
Frye, Northrop. The Secular Scripture: A Study of the Structure of Romance. Cambridge, Massachussets: Harvard UP, 1976.