Why did God need to use a proxy for the birth of his Son? According to the Bible, He didn't need to use one to create well, everything else, including the first humans. So, why use Mary at all? Any Mary couldn't have been a virgin, because she was married to Joseph. And since the Bible says that women must be virgins when they are married (or else they be stoned to death)...
Deuteronomy 22:20-21
King James Version (KJV)
20 But if this thing be true, and the tokens of virginity be not found for the damsel:
21 Then they shall bring out the damsel to the door of her father's house, and the men of her city shall stone her with stones that she die: because she hath wrought folly in Israel, to play the whore in her father's house: so shalt thou put evil away from among you.
Deuteronomy 22:20-21
King James Version (KJV)
20 But if this thing be true, and the tokens of virginity be not found for the damsel:
21 Then they shall bring out the damsel to the door of her father's house, and the men of her city shall stone her with stones that she die: because she hath wrought folly in Israel, to play the whore in her father's house: so shalt thou put evil away from among you.