Maaaaaaaaaaany, many, many years ago I had it explained to me.
The details are a bit fuzzy with time, but basically, I received a detailed run down of Mary and Josephs family tree.
Mary was from Davids line, (on the servant side of the household), but Joseph was not of Davids line. To fulfill the prophecy, Jesus had to come from Davids line.
Again, if memory serves, this was also a sticky point of the Jews as to whether He could be the messiah, because joseph was certainly not of David's line, and Mary being of the servant lineage was probably viewed as a weak link.
If I've messed this up, i'm happy to be corrected.
That she WAS a virgin at Jesus birth has been shown beyond refute scripturally in this thread. Also that she didn't remain a virgin after His birth is scripturally without question. Not to mention what would common sense tell you?
What the Catholics have done to the poor woman is reprehensible. She was God's vessel, used to achieve His ends. At the end of the day, aren't we all?
I must hunt down the family tree again, as i did find it really interesting at the time.