Why the Virgin Birth? 

"Our human nature was very much like a polluted stream up until the incarnation.  Imagine a ship, for example, sailing in polluted waters. It wishes to sail in clear waters, but without the pollution coming from one into the other.  How could the transfer of the ship be made except by  a lock?  So the ship in the foul waters would be put into a lock where there would be a separation of waters, then the ship would be raised to the level of the unpolluted waters.  Now the immaculate conception and the virgin birth were that lock.  The pollution stopped because there was no union of man and woman.  It was simply woman alone who gave a human nature to Christ and began the new humanity."