As a Christian I stand upon the Word of God as the sole authority for all of life's faith and practice. There is no room for debate or doubt when we consider what the Bible says concerning the birth of our Lord.
Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel. (Keep in mind that Isaiah gave this prophecy some 700 years before the birth of Christ the Lord. Now let's fast-forward to the events surrounding the birth of Jesus.)
Matthew 1:18-25 Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.  Then Joseph her husband, being a just man, and not willing to make her a publick example, was minded to put her away privily.  But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost.  And she shall bring forth a son, and thou shalt call his name Jesus: for he shall save his people from their sins.  Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying,  Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.  Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife:  And knew her not till she had brought forth her firstborn son: and he called his name Jesus.
Clearly we see this prophecy carried out exactly as the Lord had promised. Mary, who was espoused, or engaged, to Joesph was found with child, before they came together. This is speaking of her being found with child prior to any physical relations with Joseph. She who was a virgin was found with child. What we need to understand and embrace is this was no ordinary child; this was the only begotten Son of the Father. Mary conceived, in her virgin womb, by the Holy Ghost. V. 25 points out another important fact. After discovering that Mary was with child of the Holy Ghost, Joseph refrained from any physical contact or relations with Mary until after His birth.
The Bible is as clear as anything could be regarding the virgin birth of Christ. If we can't accept the virgin birth, we can't accept salvation of the Lord. Had Jesus not been born of a virgin, our faith would be in vain. He would have been as any other child, and in essence grown up as nothing more than a mortal man. He would not have been sinless, perfect, and holy had He been conceived of a man.
Simply put, if there were no virgin birth, there is no Savior. We cannot separate the two; they are intertwined in the deity and sovereignty of God. If Joseph were Jesus' father, He too would have needed a Savior. He would have been born is sin as all mankind is. It is impossible to reject the virgin birth and still claim Christ as Savior and Lord.
I rejoice that this isn't as issue that I struggle with. I believe without any doubt or hesitation. The God who had the power to create man from the dust of the earth had the power to send His Son, conceived of the Holy Ghost and born of a virgin womb. Rather than try to rationalize it, embrace it and rejoice in the fulfillment of God's salvation plan!