Apples and oranges, my friend.
Zecharius said, “HOW can this be; my wife and I are too old”. . .he doubted God’s power. He was sure this COULDN’T happen, scientifically speaking and so he came right out with it. . .a challenge: Just HOW is God going to make an old man and old woman, past the age of childbearing, have a son? It isn’t possible!
But Mary’s case was different. Since she was betrothed to Joseph, IF this was going to be a case of a ‘normal’ Jewish marriage, she would have EXPECTED to have sexual relations. Remember, the angel at the time had not said the child would be the Son of God–that came AFTER. No, if Mary had expected to be married and have relations, she would not have said, "how’ but rather, “oh. . .Joseph and I are going to have a SON?”
Instead, she says ‘how.’ She isn’t stupid. . .she knows that ‘a man’ is required, she isn’t asking for biology lessons.
Obviously, Mary (as Tradition tells us) was expecting not to have relations. . . though she was going to be married. That could only indicate that Mary was a professed virgin (and she told the angel that, that she was virgin) and EXPECTED TO REMAIN SO. . .so SINCE she was going to be married, it must have been a marriage to ‘protect’ her (unmarried women in 1st century or so Judea, a country that was occupied by Rome, would have been in great danger of rape and death) and it must have been accepted by the man in question as well.
And that explains Joseph’s later actions. Knowing that Mary was PREGNANT, he knew that marrying her would mean that he would not only be the ‘legal’ father of another man’s child, he would ALSO (and unfairly in his eyes) be considered as having ‘reneged’ on his vow to protect her perpetual virginity. While this would have been permitted him as her husband in theory, in practice, once he had agreed to let her remain virgin, ACCEPTING her decision, by Jewish law he would be looked down on if he ‘changed his mind.’ She would not be blamed so much but HE would be, by those he knew.
Of course if he divorced her quietly, she would have to go away someplace. . .and the talk would die down in time. If he ‘kept her’, there would always be the snide remarks, being looked down on. . .how that must have hurt this innocent man!
So Mary. . .knowing Joseph had agreed to her vow. . . was puzzled. If the angel is telling her that she will conceive–does that mean God is DISREGARDING her --and Joseph’s–Vow? She was sure she was right to vow. . .but now it looked as though God was telling her she was wrong. . .so she was puzzled. How could God tell a virgin she WOULD conceive, unless she was no longer to be a virgin? And so, Mary asks. . .How shall this be? I am a virgin (a vowed virgin). . .I do not (or so I had planned) have relations with a man. . .What do you want from me, dear Lord? Must I deny my vow? Is that your wish? Tell me. . .I will disregard my own dearest wishes if you desire. . .I am the handmaid of the Lord.
The angel explains. Mary understands. . . this is the fulfilment of the Messiah prophecy. She knows all it will entail. She has no hesitation. . .
Be it done to me according to thy word.