Bible teachers use the word, incarnation (which means in-fleshment), to refer to the fact that the Son of God, who was already God, also became a man. This does not make the virgin birth unnecessary, but rather requires it. If Jesus had been born like other men, then He could not be both God and man. Please consider the following scriptures:
Isaiah 9:6 - For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
This prophecy concerning the coming of Jesus Christ says that the "child is born." This refers to His humanity in being born as other men. But it also states that the "son is given." That which is given as a gift already exists. This points to His pre-existence and alongside the titles bestowed on him in the verse points out the fact that He is also God.
John 1:14 - And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
A simple study of the passages that use the Word as a title proves that this is Jesus Christ. Here we see that He was made flesh. He already existed as God the Son, but now He is made flesh. That is exactly what the word incarnation means. Here are some other verses that teach the same thing:
Galatians 4:4 - But when the fulness of the time was come, God sent forth his Son, made of a woman, made under the law,
1 Timothy 3:16 - And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.
Romans 8:3 - For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh:
The Son was sent (Galatians 4:4) but in His incarnation He was made of a woman. God was manifest (made known) in the flesh (1 Timothy 3:16). God sent His Son (who already existed) to take on the "likeness of sinful flesh" (Romans 8:3)--His flesh was like sinful flesh but was not sinful. But to have one who was both God and man at the same time required a different beginning; it required the virgin birth.
Luke 1:35 - 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.
The angel told Mary that the Holy Ghost and the Highest (certainly a reference to the Father) would join in power in order to bring about the birth of a son who was the Son of God. He was not to be born of a natural birth but of a supernatural one. This the only way such a person could be born on the earth.
These doctrines are so complex that many books have been written on them.