-A8
"Therefore, Mary's undergoing the rite of purification does not show that Mary was impure or unclean."
That would be true if she did not not deliver a male child in birth. Her having conceived seed, and born a man child, by the Law, rendered her impure and unclean. Jesus was sinless. His baptism was in obedience to the Law in that He identified Himself with sinners so as to become the "sin bearer" and as evidence to John that Messiah had come.
John 1:29, "The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world. This is he of whom I said, After me cometh a man which is preferred before me: for he was before me. And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.
32 And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him.
33 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost. And I saw, and bare record that this is the Son of God."
The pain and sorrow of childbirth was the judgment for sin.
"Unto the woman he said, I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children..."