In Luke 1:26-33, the angel Gabriel informs Mary that she will give birth to a child. Her response in verse 34 is quite interesting:
"'How will this be,' Mary asked the angel, 'since I am a virgin?'"
Notice that Mary's self-description of her virginity is in the present tense. She doesn't state that she will forever remain a virgin. On the other hand, Gabriel's declaration is in the future tense. Why, then, does Mary not just assume that in the future she will conceive in the natural way? Catholics, Orthodox, and some Protestants (but most Protestants today do not hold to this) believe that Luke 1:34 is indicative not only of Mary's present state, but also of her vocation.
The awkwardness of Luke 1:34 for the contradictory view is manifest in the gratuitousness of Mary's question. Where e = evidence (in particular, the passage we are discussing), h = the hypothesis that Mary remained a virgin throughout her entire life, and k = any background knowledge, P(h/e&k) > P(h/k). In other words, Luke 1:34, while not necessarily demonstrative evidence of Mary's perpetual virginity, does make this hypothesis more likely true than it would be without Luke 1:34.