Through human interaction no doubt have we seen small "white lies" and larger lies covet the air around us. From the simplest "Yeah, of course I'll be there! I'm on the highway right now!" (but is really at home watching TV) to the dreadful but common "Of course I've never cheated on you", or some form of that lie. Just this simple idea of "Lies" exists around us, if it didn't we wouldn't be human.
Apart from Biblical statements and "witnesses" that they claim the magical powers of Jesus Christ, If that man did exist, and how could he be in existence if she indeed was a virgin. You may already see a few Lies involved in this matter:
Scenario 1: Mary and her husband full blown lied to get the attention, or maybe they lied to the idea of intercourse as if it was a forbidden motion until some occurrence of marriage or other "signs" people believed in back then.
Scenario 2: Mary lied to her husband about being a being impregnated by an Angel but committed adultery with another, or maybe even got raped. Sure its a big step to actually say this, but in reality, is it? The human mind, urges, and needs, do not change with time, especially in the lower part of the brain, the "instincts" where sexual intercourse lies until it is unleashed. So what can be the chances of this? And of course Divorce and such back then was not as easy as today, and the religious punishment for many of one cheating on their husband at most was death. So with such fear in ones mind it doesn't seem all that unlikely to lie for ones own protection, Hell, we see this all the time in real life.
In a tribe located in Africa, there was a girl about the age of 15 who was savagely raped by 3 men. When she went to the village to tell of the pain she was through, in the end the villagers ended up stoning her to death because she has lost her "purity". The men that had raped her were not punished. In this sense, it is possible to see the fear and the protection of the lie as would in a primitive society.
If either of these were the case, take it that a lie such as that changes the world in such a large scale, through wars and beliefs. Thats one Damn big lie.