If you put up a Christmas tree and decorate Easter eggs, you're mixing Christianity with paganism.
We know that Mary was sinless because the angel Gabriel declared her so ("full of grace"). Man fell from grace by Original Sin. Grace was unattainable. Man could not enter heaven. The prophets were sais to rest "in the bosom of Abraham". Logic thus dictates if the angel Gabriel was being honest in calling Mary "full of grace", then she was saved from Original Sin by her saviour, Jesus Christ, at the time of her conception ("The Immaculate Conception").
The ark of the covenant carried the presence of God in the Old Testament. Mary carried the presence of God in the New Testament. Take note of the painstaking detail God commands of His people in building the ark (then the Temple later on) where He would reside. Chapter after chapter of exact measurements and decoration and procedure. Now tell me the Lord would gestate in any old womb of any old girl.
If the ark of the covenant was pristine, what then of the mother of Jesus Christ - God Himself?
Just because it doesn't say "SHE WAS SINLESS!" in bold letters, doesn't mean it's not scriptural. Just as nowhere do we find the word "Trinity" in the Bible, and fought back heresy after heresy questioning the Divine nature of Christ with the traditional understanding gained by the Magisterium, most Protestants agree that the Trinity exists as God the Father, God the Son, and God the Holy Spirit.
Just as the Bible does not contain the admonition, "create a Bible", Protestants accept the Wisdom of the Church in determining that Matthew, Mark, Luke, and John are the only Divinely inspired Gospels among the thirty or so that circulated before the canon was compiled. And this was determined not by citing the letters of Paul, but through Old Testament writings and oral tradition passed on by the Church patricians leading up to the Council.
These are historical facts which cannot be denied.
"If you put up a Christmas tree and decorate Easter eggs, you're mixing Christianity with paganism." True story, but there is a huge difference between doing something like this and teaching/asserting that Mary was sinless.
"We know that Mary was sinless because the angel Gabriel declared her so ("full of grace")." To declare someone "full of grace" is not synonymous with being sinless. Stephen (in Acts) was a man full of grace also - I assume you believe he is sinless also? And by the way, where exactly does it say that Mary was "full of grace" in one of the 27 books of the New Testament?
You posted: "We know that Mary was sinless because the angel Gabriel declared her so ("full of grace")."
How do you know that this was so throughout the rest of her life, or was true at this instant in Mary's life? In the O.T. we see many examples of men the had the Spirit of God and then did not have the Spirit of God. 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 one point in time, and then sins, so that only through Confession can they once again be close to God.