Mary did not have sex prior to the incarnation.
What does Mary having sex after the incarnation say about anything?
Is not sexual relations within the confines of Marriage sanctified by God? Is it somehow that you believe Mary was God's wife and therefore would have been committing adultery by having relations with Joseph? I do not understand why anyone even came up with the idea that Mary and Joseph having a normal relationship would somehow sully her. Unless it is from the Medieval church's dislike of sex for anything other that procreation.
Hebrews 13:4 (Douay-Rheims 1899 American Edition.)
Marriage honourable in all, and the bed undefiled. For fornicators and adulterers God will judge.
This is a unique situation and has nothing to do with the sanctity of Marriage. The virginity of Mary not only expresses the singular and sublime work of God in the incarnation (and the vessel He worked through), but also expresses something about the Church itself, since Mary and the Church are interwoven throughout Scriptures. As I have said here before, what the Church says about itself can also, in a spiritual sense, be said about Mary. Thus, the idea that Mary REMAINED totally committed to God says something about the Bride of Christ.
Regards