Mary was the surrogate mother of Jesus the Christ. She in no more the mother of God than Mother Teresa was. God placed the body of our Lord in the womb of Mary.
HEB 10:5-6 Therefore, when Christ came into the world, he said: "Sacrifice and offering you did not desire, but a body you prepared for me; with burnt offerings and sin offerings you were not pleased.
Not a sperm cell to unite with Mary's sinful nature (all have sinned), but a body prepared by God. God prepared the body of Adam, and he also prepared the body of Jesus. If you believe that Jesus was half Mary, and half God then Jesus was not the perfect Lamb of God, but of course we know that he was all God, and that it was a perfect Lamb of God that died on the cross to redeem us from our sins.
1CO 15:45 So it is written: "The first man Adam became a living being"; the last Adam, a life-giving spirit.
46 The spiritual did not come first, but the natural, and after that the spiritual.
47 The first man was of the dust of the earth, the second man from heaven.
48 As was the earthly man, so are those who are of the earth; and as is the man from heaven, so also are those who are of heaven.
49 And just as we have borne the likeness of the earthly man, so shall we bear the likeness of the man from heaven.
This is how the Catholic theologians got off on the wrong track. They reasoned; if Jesus was without sin, then Mary had to also be without sin, and they took their false doctrine from there to where they are today. Don't forget it was the woman who first sinned in the Garden of Eden, so don't say the man's sperm carries the sin nature, because Eve was the first to sin against God.
1TI 2:13-14 For Adam was formed first, then Eve. And Adam was not the one deceived; it was the woman who was deceived and became a sinner.