The atheists were first with their sign on one end of the Lincoln Tunnel which reads: You know it's a myth--this season, celebrate reason. The Catholic League bought a billboard at the opposite tunnel entrance; it reads: You know its real. This season, celebrate Jesus.
It's unfortunate that we're supposed to take these billboards seriously, as both overlook the deeper meaning of the Christmas story, the people involved, and the nature of revelation. I assume that the atheists are referring to the virgin birth being the myth, as the person of Jesus Christ is part of the historical record. What is unfortunate about the Catholic Leagues response is that you can't do serious theology on billboards, and therefore, challenging the the assertion of the atheists in such a way only continues to dumb down what may be the deeper meaning of the assertion that Mary was a virgin.
The modern question of Mary's virginity seems to be one of whether or not Mary was chaste before Jesus' birth, it is a biological or medical question, often resolved on the basis of physical technicalities. But this question, which seems to enthrall everyone, especially this time of year was not the question that is answered by the biblical assertion that Mary was a virgin.
The angel of the Lord who visited Joseph in the gospel of Matthew declared (in the words of Isaiah the prophet to Ahaz the king of Judah) that "a virgin shall conceive and bear a son..."
One needs to consider what this means in the context in which Matthew was written. Mary and Joseph lived under the oppressive Roman Empire; an empire that embraced pater familias, which was both a system for tax collection, and a system in which patriarchal authority, especially that of the emperor ( who named himself Dominus Deus, Lord and God) was paramount. Within this system, virgin daughters (chaste daughters) were regularly given in marriage to essentially the highest bidder. Unchaste daughters were much harder to sell. Mary as described in Matthew and Luke does not fit this patriarchal pattern. First, we hear nothing of Mary's father in gospel accounts. She is her own person. In fact, when she visited her cousin Elizabeth, the one person who might have played such a patriarchal role, Zechariah, has been silenced by the angel Gabriel for his lack of belief in the announcement of John the Baptist's birth. Joseph, for his part hardly takes on the role of patriarch in any oppressive sense, with the angel visitation he assumes the role as obedient servant of God and of Mary.
Much as we might not see it (and perhaps wish not to), Mary's role as virgin is much more akin to the role taken on by women in other ancient religious cults who present themselves to the deity for sexual purposes. Now we may blanche at this, but the Greeks understood that those who had divinity were usually born of virgins.
The virginity of Mary as expressed by Matthew and Luke is actually not just a challenge to the paternal system, but a challenge to Roman imperial authority. In the Magnificat, Mary expresses her gratitude at being chosen, but her words also reflect that the God she has given herself too is a God who undermines what Rome stands for.

"And Mary said,
'My soul magnifies the Lord,
and my spriit rejoices in God my
Savior,
for he has regarded the low estate
of his handmaiden.....
He has shown strength wih his arm,
he has catteered the proud in the
imagination of their hearts,
he has put down the mighty from
their thrones,
and exalted those of low degree;
he has filled the hungry with good things,
and the rich he has sent empty away.'"

Her words, spoken in the context of Roman oppression are revolutionary. It should come as no surprise that Mary was given the title Theotokos, God Bearer, and has been named the first believer in Jesus Christ. Some Icons depict Mary standing at the very center of the Apostles. Whether in icons or stained glass, Mary has never been depicted as the virgin (chaste) daughter of a male patriarch; her virginity was much more than this, it was indeed a challenge to the cult of the emperor and all that Rome stood for. And if Mary's story is going to be our story, we need to be certain that we don't shrink her down to the point where she is a mere pawn in the story of the incarnation.
Some of this you may hear again in Sunday's sermon, but then, we have to look at Joseph, the Angel of the Lord, Isaiah, Ahaz, Herod and the rest, and figure out how this story is our story too. Forget the argument taking place at the Lincoln Tunnel, we have a serious story to make our own, to tell, and to celebrate.