Why indeed? What was the need for a virgin Mary? What was so great about god providing a virgin with a fetus anyway? What purpose did it solve? In fact it only created more problems for Jesus and his supporters.
For one, people do not believe in the virgin birth. Even if I believe that someone called Jesus did walk around the land of Israel preaching, I cannot believe he was born via parthenogenesis. I mean, no one checked Mary to verify her claim. She was married at the time and that too puts a dampener on things. So why did god need to send his “only begotten son” through this tortuous, hard to believe and even harder to defend route? And what sins had Joseph committed that he was condemned to marry an already pregnant girl?
Jesus was supposed to be man’s saviour (Women too? Are we sure about that?;)). So why make it so difficult to believe in him? That is the only route to salvation after all, believing in Jesus. The easiest thing would have been for god to make a grand entrance in the Great Temple of Jerusalem. All clouds and fire and what not and in the middle of it all, an infant descending through the air to land softly in the middle of the complex. Now that would be a son of god no one would ever doubt! Imagine how many more people could be saved if honest Christians could claim such a glory-born saviour! Does god hate mankind then, that he has made believing in his own symbol of salvation such a difficult task?