The angel shows up and tells her how it’s going to be.
Mary’s stating May your word to me be fulfilled.” is NOT an acceptance of a choice given her.
Gab was relaying a message from GOD: period.
For the sake of balance, Paul in conversion was shown by the Lord "how great things he must suffer for My name's sake," (Acts 9:16) and accepted it, and labored more abundantly than the other apostles, by God's grace, and brought forth the word of God more than all others save Moses, nor is any sin assuredly ascribed to him.
See http://triablogue.blogspot.com/2012/08/51-biblical-proofs-of-pauline-papacy.html for more (spoof).