This is not the birth one would expect for God, is it? Born to a young girl already pledged to another. I think we overlook that a lot. Mary was promised to another man. Why not choose a virgin who was not already promised? One reason I believe is that it gave false legitimacy to her pregnancy.A virgin who was pregnant would likely have been stoned for her adultery. With Joseph in the picture, there was some semblance of legitimacy to her pregnancy. Now, Joseph could have denied it was his and ordered the stoning, but he was a "righteous" man. But did not righteousness require her stoning? So there must be another kind of righteousness besides the one that adheres to the strict letter of the law.
Imagine God taking a woman who had already been pledged to another. But isn't that what He has done by taking us? We were pledged to death, to sin, to destruction. We were full of sin, ugly and shamed. But He takes us as His bride. Just like Hosea, he marries a whore. And He loves her.
Okay, back to the original post...
Isn't that why Jesus came? To offer a new kind of righteousness, one that fulfills the law in one sense, but rises above it in another sense. I know that in reality, Jesus was the Son of God, but I wonder too, how much this righteous, human man was just the right person to be His earthly father.
One more thought, regarding the two names of this child. One was Jesus, which literally means Jehovah is salvation. Even His name points to God. Every time the disciples called Him Jesus, they were pointing to God. Even more ironically, every time the Pharisees called His name, they too were pointing to God as the author of salvation. And if God is the author, He gets to write the book as He pleases. The child would also be called Emmanuel, meaning "God with us." The combination of these two names is amazing. God, who is the author of salvation is with us.