John 6:63 It is the Spirit who gives life; the flesh profits nothing. The words that I speak to you are spirit, and they are life.
That’s not exactly what I was trying to get at.
Keep in mind that while it sometimes happens that things have no good reason it also sometimes happens that they may.
The idea of immaculate conception might be attributed to people trying to cope with the thought that had Mary had sin then her own blood would have physically polluted Christ’s. I can even see it as having been raised to help deal with claims such as were made by Gnostics, a kind of defense of Christ’s humanity (though of course if THAT it just moves the problem from Him to His human mother).
Now, there are problem pregnancies where the placental barrier doesn’t protect the infant from the mother’s blood, or even from a twin’s as happened to my baby brother, but those are exceptions. Because the baby may well have a different blood type than the mother these can result in miscarriages. My brother barely survived the small amount of blood he got from his twin sister.
A pregnancy where all goes well sees none of that. As a consequence, it would have been impossible for Mary (here assuming that the Lord would have arranged that her pregnancy went without hiccup) to have been a source of physical contamination.
So no physical/medical need was actually required to justify the immaculate conception of Mary.
BTW, this function of the placental barrier is relatively recent medical knowledge.
That leaves the legal and spiritual grounds to argue over where immaculate conception is concerned: and I attempted to argue this too would also likely not be an issue if the spiritual nature is inherited from the father rather than the mother.