"...for all have sinned" (Rom. 3:22)
...with the exception of Jesus, so why not also His Mother, Mary, the One Who conceived Him, God Incarnate?
There's no scriptural verse that forbids the exception applying to Mary as well.
Jesus referred to Himself as God in various ways more than once (Jn. 8:58;10:22-36), even the Pharisees understood that He did, which is why they told him they were going to stone Him for "blasphemy" for "making Himself God." (Jn. 10:33)
Now, consider that only the High Priest was allowed into the Holy of Holies to offer sacrifice to God for the sins of humanity.
Would Mary not have to be so Holy and Perfect, to the point of being Second only to God, as to conceive God Incarnate, the Messiah, and offer Him to God the Father as a sacrifice for the sins of humanity? Could God, in advance, not have Thought of creating the soul that was to be the soul of the Mother of God Incarnate, preserving it from inheriting the stain of original sin, and thus becoming an immaculate soul capable of possessing the fullness of His Grace, then infusing it into the embryo at the moment of conception? Could these factors, coupled with being conceived by and born of two Just human parents and having a natural good will, not result in Mary being a worthy dwelling place for God Incarnate, the Son of God, the Messiah, the Savior, the most Holy and Perfect One, to be conceived and take form in, and live a divinely Holy life with in Spirit and Body on earth and in Heaven?
Peter says, “...like the Holy One who called you, be holy yourselves also in all your behavior; because it is written: “You shall be holy, for I am holy” (1 Pet. 1:15-16, cf. Lev. 19:2;20:7;20:26;21:8). Jesus, God Incarnate, was holy in all His behavior because He was without sin (1 Pet. 2:22), and thus He was completely Holy. If Mary was not without sin in all Her behavior, then She was not completely Holy and Perfect. Sin is disobedience, evil, and unholy (1 Jn. 3:4;8), and "no evil can dwell with God" (Ps. 5:4). Therefore, how could God, the most Holy and Perfect One, allegedly create a soul, and as God Incarnate take form and dwell in a body, that was not the most Holy and Perfect so as to be Second only to Him?