I don't remember being taught that Mary was sinless. Just a vigin. And then "Holy" (After giving birth to Jesus, I guess? Or maybe before...And that's why she was chosen by God?)
Anyway...I don't remember anything about her being without sin.
??
Mary's Sinlessness has to do with the Church's interpretation of "full of Grace" (which I'm sure you would disagree with) and the Tradition handed down by the Apostles (which you would also disagree with).
From http://www.catholic.org/clife/prayers/prayers.php?section_id=31&name=Marian%20(Mary) - which I assume to be the official Catholic website. I could be wrong about that - anyone know?
The trick question of catholic doctrine is who is the only person born without original sin? Adam was created by God, Eve Shaped out of his rib and therefore only Mary was born without original sin, to be the mother of the Son of God. Jesus was born with original sign and baptized.