Why A Virgin Birth

Luke 1:26-35 KJV
(26) And in the sixth month the angel Gabriel was sent from God unto a city of Galilee, named Nazareth,
(27) To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary.
(28) And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women.
(29) And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be.
(30) And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
(31) And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
(32) He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
(33) And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
(34) Then said Mary unto the angel, How shall this be, seeing I know not a man?
(35) And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.