OK so it finally took a priest to explain this last question to me. Not a single Catholic-raised person I have asked so far had any idea that this was the official story. Either the priest was totally bullshitting me, or the Holy See has done a good job of keeping this story fairly under wraps.
Apparently, the Immaculate Conception does not refer to when Jesus was, well, immaculately conceived. It refers to the moment of Mary's conception.
Well if Mary was immaculately conceived too doesn't that make Jesus' birth less special and miraculous? What if parthogenesis just ran in the family? Now wait for it, Mary was not actually immaculately conceived. She was brought to be with a good old fashioned romp in the haystack. However, God interfered at this quite private moment and made sure that the normally-conceived Mary would not be born with original sin. How very thoughtful of him.
And what passages of the Bible was this complicated convoluted story inferred from exactly? Enough with the fucking questions! Just take your four day weekend and enjoy it!
Oh well, don't mind if I do :)