So just WHY isn’t MARY a sinner like the rest of us?
I think you can get more of the Big Picture by linking Genesis 3:15 (God says to the Serpent: "I will put enmity between you and the woman, between your seed and her seed...") with Hebrews 10:5 ("Therefore when He came into the world, He said,...a body You have prepared Me.")
God was working through all the generations between Adam/Eve, and Mary, preparing perfect seed so that Our Lord could be born of a perfect mother --- because He had to have a perfect human nature prepared for Him.
Being 21st century persons, we naturally think of this in terms of the DNA, and the various permutations and combinations which come about, generation after generation. The OT Jews didn't know genetics, but their intense interest in genealogy in both the OT and the NT suggests that with each generation, God was preparing the sacred Messianic line.
But why do we bump that back a generation, to the conception of Mary in her mother's womb? Why not simply say that Jesus Himself, upon His conception, must have received those "all-star" genes when the finally-flawless sequencing clicked into place, making Him the perfect human being??
The reason would be that be that there are always interchanges between maternal and fetal blood involving stem cells and proteins which switch different genes on and off. A perfect Jesus carried in the womb of an imperfect mother for nine months, would have been damaged by processes (which science is only now learning about), while He was still in her contaminated, and actively contaminating, womb.
Not only that, but Mary's egg cell (which, when fertilized, became the zygote Jesus) also contains mitchondria, which have their own independent genome, and are involved in other tasks in the cell cycle, such as signaling, differentiation, growth, and death. This is another event pathway by which an imperfect mother would inevitably degrade a perfect offspring.
There's also the largely unexplored question of living cells transmitted to the suckling infant via breast milk. The mother passes on 40 million maternal cells per ounce of breast milk: white blood cells, macrophages and so forth. These are the same cells found in the mother's blood. Again, an imperfect mother would be actively transmitting her corrupt living cells into her perfect Offspring.
Bottom line, modern science can be seen as confirming what our fathers in the faith knew only by divine inspiration: that the "seed of the woman" (unheard of for a woman to be thought to have "seed"!) would need to be "prepared" (as it says in Hebrews) until you had this ---well, as the angel said, Kecharitomene. A woman filled with grace from the beginning, with a perfect human nature --- as perfect as Eve was, before the catastrophe of sin --- who would give birth to the Messiah.
BEFORE YOU GO; OH, WOO WOO, CELLULAR BIOLOGY??
I don't know how much of theology of Jesus' perfect humanity is dependent on the exact biology as I have just described it. Genetics is not dogma. (Remember that I said that!)
But we do know that genetics plays THE defining role in the transmission of a human nature, and it's human nature that needed to be perfect, without all the defects, the darkening of the mind and a damaging of the will, which inclines us to sin, and which we otherwise inherit from Adam and Eve.
Much of it must necessarily remain veiled in mystery. Nevertheless, science provides converging lines of evidence that the mother would have to be perfect in order to conceive, gestate for 9 months, and give birth to an offspring entirely free of defect, and nurture him perfectly through infancy.
As one the most ancient hymns in Christendom says, "[You] who without corruption gave birth to God the Word, the very Theotokos, we magnify you." And all this, done for a lowly handmaid, by her Savior! It was a work of God alone. He prepared her body and soul. Then she heard the Word of God, and kept it.
"My soul magnifies the Lord!"