Mary would not have said no.
Mary was full of grace, completely conformed to the will of God.
Mary’s will was the same as God’s will, that the Second Person of the Trinity be brought into the world by her.
Anyone perfectly full of grace, perfectly conformed to the will of God cannot say no to something God wills that he do.
-Tim-
Let me quote this article from
boston-catholic-journal.com which led me to asking my question:
"The Unum Necessarium (the one thing necessary)
Mary is the one person ever to contribute, to truly give, the one thing to God that was not already His, even as He first imparted it to her. “The earth is the Lord’s and the fullness thereof: the world, and all they that dwell therein.” (Psalm 24.1)
It was something necessary to the final and perfect fulfillment of the will of God. In fact, it was the one thing that God created but did not possess. Apart from it, the suffering, crucifixion, death, and resurrection — absolutely necessary to the fulfillment of God’s will for the salvation of the world, for the redemption of souls from bondage to sin and death — was impossible: her very flesh! Mary assented to the will of God: “Et Verbum caro factum est”, “And the Word became flesh!”
Jesus Christ took the substance of His sacred humanity from Mary. It was in this Sacred Humanity that Christ preached, healed, raised from the dead, gave sight to the blind. It is also in His Sacred Humanity that Christ suffered, was crucified, and died for our sins and through which He purchased our salvation. Had Mary not consented to the will of God; had she refused to be the Mother of God’s Son (Who Himself is One with the Father), the one thing absolutely necessary to our salvation — the flesh and the humanity which Jesus Christ assumed, and through which alone salvation came into the world in the Person of Jesus Christ — could never have been possible.
“God is a Spirit” (St. John 4.24), and spirit is not possessed of flesh together with all the limitations inherent within it. God is infinite. Flesh is not. God is everywhere present, flesh is not. God is perfect felicity, which is to say, God in Himself has ever been, is, and ever will be, perfectly happy, unassailed by suffering, and pain cannot touch upon Him — but flesh is not! For this reason, Saint Paul tells us that Jesus Christ, “Who being in the form of God … emptied Himself, taking the form of a servant, being made in the likeness of men.” (Philippians 2.5-7). How? Through the Incarnation. Through whom? Through Mary who contributed her flesh (giving Christ, Who had emptied Himself of “the form of God”, the “form of a servant”; in fact the “Suffering Servant” of Isaiah 53 through Whom mankind was redeemed. Only in the humanity that Christ took from Mary alone, could He possibly suffer … even die!
Theologians speak of this in terms of the “Hypostatic Union”, or the union of God and Man in the Person Jesus Christ. Christ is both! But it was in His humanity that He suffered and redeemed the world — the humanity, the flesh, given Him by Mary alone. In her assent to the will of God, in her “Fiat mihi secundum verbum tuum”, “Be it done to me according to your word” (Saint Luke 1.38), she, the lowly “handmaid of the Lord”, gave to God the one thing that Spirit does not, cannot, possess: flesh. Her flesh — which the Son of God assumed, becoming through her assent, “True God and True Man.”
I find the notion of Mary freely oferring something that God had created but not yet possessed especially intriguing!
