As I continue with my study in Luke and have reached the annunciation in my commentary I realise the clue is in the Greek. Not a verb tense but the prose. Jesus is described as ascending to David's throne. Here in the UK we occasionaly see an article about the sixteenth in line to the throne. Mary was not a noblewoman her son would not be "next-in-line" yet he is described as sitting on David's throne?? Likewise Jesus is described as the Son of the Most High, not a child of God but "THE" Son of God. Then there is the eternal nature of Jesus reign. This was not the plan she had for her children with Joseph.
When she replies her focus is not on her virginity but that she "knows not a man". She knows that storks don't deliver babies and can also say that she has not been with a man. Her confusion is understandable, no doubt she was looking forward to children and hoping they would have a good job (as carpenters) marry well and have lots of granchildren. Gabriel is mapping out a very different future for her son Jesus.
When she says how can this be, she is responding to everything but focusing on the fact she does not yet have a father for her children.