why did Mary ask the angel “How can this be, since I know not man?”
When you think about it, it seems that if Mary was expecting to consummate her marriage with Joseph, her question would have been when would this be, not how can this be.
We don't know how far Mary and Joseph were into their "betrothal" period - which was usually one year before consummation. My thinking is the angel Gabriel's words were that the conception was imminent which would cause Mary to ask her question about not "knowing" (in the Biblical sense) a man. I also believe religious Jews would be familiar with Isaiah's prophecy that a "virgin will conceive and bear a son and call his name Immanuel." (Isaiah 7:14).
Perhaps she was thinking about the many things mentioned in verses 31 thru 33?
27 To a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary.
28 And the angel came in unto her, and said, Hail, thou that art highly favoured, the Lord is with thee: blessed art thou among women.
29 And when she saw him, she was troubled at his saying, and cast in her mind what manner of salutation this should be.
30 And the angel said unto her, Fear not, Mary: for thou hast found favour with God.
31 And, behold, thou shalt conceive in thy womb, and bring forth a son, and shalt call his name JESUS.
32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David:
33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
34 Then said Mary unto the angel, How shall this be, seeing I know not a man?
35 And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
36 And, behold, thy cousin Elisabeth, she hath also conceived a son in her old age: and this is the sixth month with her, who was called barren.
37 For with God nothing shall be impossible.
38 And Mary said, Behold the handmaid of the Lord; be it unto me according to thy word. And the angel departed from her.
I find it amazing that the ANGEL told Mary what WAS GOING TO HAPPEN; Yet Rome can find that Mary gave her permission to it all in this verse.