Virgin Birth
After creating the entire universe from nothing, when the time came, it was easy for God to create one sperm and put it where it would do the most good. So, all of us who accept the fact that God was the un-caused cause of the universe, can easily understand HOW God accomplished virgin birth.
Much more interesting is WHY virgin birth. I believe it was primarily for the effect it had on the Virgin Mary. God asked a great deal of Mary. She willingly, lovingly gave God her body and her life to use for His purposes without knowing exactly what His purposes were. But she wondered. As the Bible says, she pondered.
Virgin birth gave Mary facts she could build her faith on. Mary knew, absolutely knew, Jesus was her son. (Women know such things.) AND Mary knew, absolutely knew, she was a virgin. (Women know such things.)
The testing and consequent strengthening of her faith came to a climax with the crucifixion of Jesus. The Bible does not tell us Mary had any special forewarning of the crucifixion. She may well have expected Jesus to be a conquering king like the Apostles expected. His crucifixion seemed like His total defeat, yet how could God be defeated? All His disciples, including Mary, were bewildered and frightened.
The crucifixion of a leader like Jesus would commonly be followed by the arrest and crucifixion of all known followers. So during the time Christ lay in the tomb, the Apostles and Mary hid together.
I believe Mary held the Apostles together through those long hours of confusion and fear. She probably could not answer questions of why God let Jesus be crucified. But she could assure them that she knew, absolutely knew,that Jesus was indeed The Son of God.
Virgin birth gave Mary absolute knowledge that yes, Jesus was her son, and yes, she was a virgin. This allowed her to stay calm in her bewilderment. Thus, she was able to calm the Apostles and keep them together until Jesus revealed His resurrected self.
This is part of how and why God used virgin birth.
(Please note: in most of this site I present the teachings of the Roman Catholic Church as I understand them. This page however, presents my own ideas. The Church may or may not agree with me.)