Why a Virgin Birth?

http://www.macarthurcommentaries.com

The astonishing announcement from an angel of God that she was to be the mother of the long-awaited Messiah (1:30-33) left Mary shaken and confused (v. 29). Overwhelmed by the implications of his announcement, and wondering how it would be practically implemented, she asked Gabriel, “How can this be, since I am a virgin?” The thought of having a child without the impregnation of a man was inconceivable. But Mary's questions did not reflect doubt or incredulity (unlike Zacharias's [1:18-20]); she believed what the angel told her, but did not understand how it could happen.

It must be remembered that miracles were extremely rare in history, as the record of the Old Testament shows. By Mary's day there had been no divine revelation or miracles for four centuries. No one had seen an angel during that time either, until Gabriel's appearance to Zacharias (which Mary probably did not know about since the angel had to tell her about Elizabeth's pregnancy [v. 36]) a few months earlier. Mary realized that the angel did not mean that she would become pregnant naturally, after she consummated her marriage to Joseph. She understood that he was saying she would become pregnant while she was still a virgin; her question was not an expression of doubt, but a request for an explanation of the means for that impossibility...