St. Bonaventure has an interesting take on the birth of Jesus Christ. He says that there are four ways of producing a human being:
1. Someone could be born from neither a man nor a woman, as was the case with Adam, who came directly from God.
2. Someone could be born from a man, but not from a woman, as was the case with Eve, who came from Adam's side but had no human mother.
3. As it is with just about everybody else, one can be born from the union of a human mother and father.
4. This leaves just one more possibility, for someone to be born only of a woman, with no human father. This is, of course, the virgin birth of Jesus.
Thus completing the possibilities for how one can come into the world, Jesus provides creation with a certain completeness and universality.
I pointed out this argument to one of my teachers once, and he said, "It's like, see how cute and clever God is." Nevertheless, it is Bonaventure's way of getting at the completion of creation in the Incarnation and at a sense of fullness of purpose for God and the world that it brings.
For the real text, see Breviloquium IV:5.