Symbolism, Imagery, Allegory
If you're like us, you're probably wondering why the play and most of the characters in it are so obsessed with Miranda's virginity. Prospero is always talking about it (and guarding it from the likes of Caliban) and, when Ferdinand sees Miranda for the first time, he says he hopes she's unmarried and still carrying her V-card (1.2.509-511). What the heck is going on here? We've done some investigating (read: close analysis of the text) and we've come up with some ideas.
First of all, it was really, really, really important for unmarried women to be chaste in Shakespeare's day. If they had sex before marriage, they were considered damaged goods who couldn't be depended on to produce legitimate offspring. (Trust us. There were entire sermons and books written about the subject.)
Miranda's virginity is a thing that's treated like a "treasure" to be guarded, mostly by her dad, who prevents Caliban from raping her and populating the "isle with Calibans" (1.2). Prospero not only prevents his daughter from being assaulted, he also puts a stop to the potential threat that the island could be taken over by the offspring of his slave. Prospero would much rather give his daughter over to Prince Ferdinand (although he gives his son-in-law a huge lecture about keeping his hands to himself until after the wedding) because 1) Miranda loves the guy and 2) Miranda and Ferdinand will have legitimate babies that will one day rule Naples.
At times, it also seems like Miranda's virginity is symbolic of her purity, innocence, and goodness. (As opposed to Sycorax the witch, who hooked up with the devil and gave birth to Caliban.) It also seems like Miranda's status as a virgin helps to somehow redeem the island's naturalness.
Remember that the last woman on the island was Sycorax. She was unnatural by virtue of being a witch, but also because when she came to the island, she was already carrying the devil's child (it doesn't get any more unnatural than that). If the island is to be a place of redemption for all the characters in the play, Miranda's virginity is symbolic of the promise of a new and pure beginning.
We also want to point out that Miranda will inevitably lose her virginity to her new husband and this signals that she is growing up and, well, changing in ways that not even her father can manipulate and control. Miranda (unlike Isabella in Measure for Measure) is really excited about this and says as much about what she "desires to give" Ferdinand after she becomes his wife (3.1).