WHY WAS ZECHARIAH PUNISHED FOR HIS DOUBT, WHILE MARY WASN'T?

Luke 1:18-20. (regarding the conception of John the Baptist) And Zacharias said unto the angel, Whereby shall I know this? for I am an old man, and my wife well stricken in years. And the angel answering said unto him, I am Gabriel, that stand in the presence of God; and am sent to speak unto thee, and to shew thee these glad tidings. And, behold, thou shalt be dumb, and not able to speak, until the day that these things shall be performed, because thou believest not my words, which shall be fulfilled in their season. Luke 1:34. (regarding the conception of Jesus) Then said Mary unto the angel, How shall this be, seeing I know not a man?

This would only be a contradiction if the nature of the questions being asked were exactly the same, and they aren't.  Zacharias is doubting that the conception of John is possible at all.  His question is "Whereby shall I know this?" (or a more modern translation is "How can I be sure of this?", as given in the NIV).  He's asking how he can be sure that this is going to happen (as if being told by a messenger of God that it's going to happen isn't enough).  Mary, however, is not doubting that it's possible.  She's not asking "shall this be?", but "HOW shall this be?"  She's merely asking for clarification on how, being a virgin, she's going to become pregnant.  Mary's question, unlike Zechariah's, expresses no doubt towards the truth of what the messenger is saying.  Not a contradiction.