There still would have been a need for a human birth, because if Christ was conceived naturally by Mary and Joseph, he would not have been divine. He would have been a natural human, nothing more.
Christ's conception was the moment when the hypostatic union came into being--a single person with two natures, human and divine. Mary provided the human nature, the Holy Spirit provided the divine nature.
Which, I think actually argues for the immaculate conception of Mary. Or else you're in the uncomfortable position of thinking that the Holy Spirit merged his divine nature with a human nature tainted with original sin. It's hard enough to fathom God condescending to take on flesh--but sinful flesh?