Please don't take offense but the logic of this is just absolutely stupefying. The example used is Mary. Yet Catholics tell us that Mary was protected by God from sin to be a pure vessel for our Lord. Consequently, one has to wonder why would Mary NOT be obedient to God, since God instilled grace into her so she would not sin? So even if Catholics believe that Mary was pure from birth, her obedience wasn't based upon her action or her works because she hadn't done anything nor would she ever sin because of God's grace. Instead Mary's purity was based upon God's action. That is, unless you believe that Mary was responsible for her purity. If that's the case then you're off the reservation.
God puts into us a new heart and spirit to cause us to walk in His ways and obey His commandments. Why Catholics cannot make this leap into believers' lives is the real mystery.
Mary was not protected from all sin, but only from original sin — a wholly different category.
Secondly, she could have lawfully declined at the Annunciation as any woman may. That would not have been a sin.
For purity, we do our part and God does His part, for we are sentient human beings with free will.