I have seen posted on this thread that the proof that Mary was without sin is that: "We know that Mary was sinless because the angel Gabriel declared her so ("full of grace")."
How do you know that Mary was "full of grace" continually throughout the rest of her life? Or, was this true at this instant (and other points in time) in Mary's life? In the O.T. we see many examples of men where we are told that they had the Spirit of God and then at a later point the Spirit of God had departed from them. As a matter of fact, I believe that it would be consistent with Catholic belief to say that an individual is ultimately close to God at singular points in time, and then sins, so that only through Confession can they once again be close to God.
Aquinas says, "But she would not have been worthy to be the Mother of God, if she had ever sinned. First, because the honor of the parents reflects on the child, according to Prov. 17:6: "The glory of children are their fathers": and consequently, on the other hand, the Mother's shame would have reflected on her Son. Secondly, because of the singular affinity between her and Christ, who took flesh from her: and it is written (2 Cor. 6:15): "What concord hath Christ with Belial?" Thirdly, because of the singular manner in which the Son of God, who is the "Divine Wisdom" (1 Cor. 1:24) dwelt in her, not only in her soul but in her womb. And it is written (Wis. 1:4): "Wisdom will not enter into a malicious soul, nor dwell in a body subject to sins."
-A8