What did the virginity of Mary signify?

The first thing the perpetual virginity of Mary makes clear is that the entire project of salvation is God’s initiative, not ours. Mary’s virginity manifests God’s absolute initiative in the Incarnation. Jesus has only God as Father.

 It reveals the deepest truth about Jesus, and us  - Jesus has God as His Father not as a stunt, but because this is the deepest truth about Him. And because it’s true of Him, it becomes true of us when we’re adopted by God through His grace.

Because of this, we are, so to speak, made members of a new human race headed by a New Adam (1 Cor 15:45-50). But that New Adam has a corresponding figure: the New Eve whose “yes” to God allows life to enter into the world, just as the “no” of the first Eve brought death into the world. And that “yes” is the fruit both of God’s predestining grace and of her own free assent: Thus, giving her consent to God’s word, Mary becomes the mother of Jesus. 
Espousing the divine will for salvation wholeheartedly, without a single sin to restrain her, she gave herself entirely to the person and to the work of her Son; she did so in order to serve the mystery of redemption with him and dependent on him, by God’s grace: As St. Irenaeus says, “Being obedient she became the cause of salvation for herself and for the whole human race.” Hence not a few of the early Fathers gladly assert . . . : “The knot of Eve’s disobedience was untied by Mary’s obedience: what the virgin Eve bound through her disbelief, Mary loosened by her faith.”
All of which means that Mary is identified with the family of the New Adam just as much as the old Eve was identified with the family of the old Adam.

Mark P. Shea

Crisis Magazine - Part Three of a four-part series on Mary's perpetual virginity, why it's biblical and why it matters by apologist Mark Shea. The Morley Publishing Group, Inc., February 22, 2011
Excerpt and adapted from www.catholicculture.org