I know not a man - yes, she was a virgin BEFORE the birth of Christ
nothing shall be impossible - hench the virgin conception
I agree that Gabriel is explaining the virginal conception. That isn't the part I'm disputing.
The question is why Mary's statement leads to that answer.
You say:
"she was a virgin BEFORE the birth of Christ"
But under the position defended in this thread, Mary expected future marital relations with Joseph and therefore expected future conception through Joseph.
Gabriel never says when the conception will occur. He simply tells Her:
"You will conceive and bear a Son."
So why does Mary present not knowing man as the obstacle to that announcement rather than understanding the conception as something that would occur through the marital relations she supposedly expected in the future?
And why does Gabriel reinforce his answer by appealing to Elizabeth's otherwise impossible conception and concluding:
"For with God nothing shall be impossible"?
Simply saying "virgin conception" restates Gabriel's answer. My question is how your interpretation reconciles Mary's appeal to "I know not man," Gabriel's explanation of divine conception, his appeal to Elizabeth's otherwise impossible conception, and his conclusion, "For with God nothing shall be impossible," with the claim that Mary expected future marital relations and future conception through Joseph.