What Gabriel told her was this:
30 And the angel said to her, “31 And behold, you will conceive in your womb and give birth to a son, and you shall name Him Jesus. 32 He will be great and will be called the Son of the Most High."
"He will be called the Son of the Most High."
Though she was likely confused, suppose she realized one of the consequences of what that meant. She knew right there the child would not be Joseph's. Then the question of "how" becomes very relevant, especially if she was aware of some of the pagan traditions of the Romans occupiers. She brought up her status as a virgin because she knew Deuteronomy 22:13-21 calls for the public stoning of virgins who don't pass the virginity test. Divorce and public humiliation might have been the best-case scenario.
Gabe answers the "how" question, and tells Mary to trust God.
That is an interesting proposal, but consider what Luke actually records.
Gabriel says:
"You will conceive in Your womb and bear a son ... He will be great and will be called the Son of the Most High."
From that, you are suggesting that Mary immediately inferred:
"This child cannot be Joseph's, therefore the conception must somehow be miraculous."
But Luke himself gives Mary's reason for asking:
"How will this occur, since I know not man?"
He does not say:
"How shall this be, since this child must be the Son of God?"
Nor:
"How shall this be, since Joseph cannot be the father?"
The explanation Mary Herself provides is:
"I know not man."
And Gabriel's response directly addresses that difficulty:
"The Holy Spirit will come upon You, and the power of the Most High will overshadow You."
That is why I keep returning to the same question.
If Mary expected future marital relations with Joseph, then conception through Joseph remained entirely possible.
Why, then, does She present Her not knowing man as an obstacle requiring divine explanation?
The text explicitly gives us Mary's difficulty and Gabriel's answer.
The idea that Mary had already deduced a miraculous conception from the title "Son of the Most High" seems to be an additional inference rather than something Luke actually tells us.
Even if, for the sake of argument, I granted your explanation of why Mary asked "How?", two other features of the narrative still remain unexplained.
First, why does Gabriel answer by describing a miraculous, non-sexual conception rather than addressing the particular concern you believe prompted Mary's question?
Mary asks:
"How will this occur, since I know not man?"
According to your proposal, Mary is concerned because She has concluded that Joseph cannot be the father and therefore faces the prospect of public shame, divorce, or even punishment under the Law.
Yet Gabriel does not address those concerns.
He does not explain how Joseph will react.
He does not explain how the community will react.
He does not discuss divorce, public shame, or the penalties of Deuteronomy.
Instead, he explains the means:
"The Holy Spirit will come upon You, and the power of the Most High will overshadow You."
That is an explanation of
how conception will occur, not an explanation of the social or legal consequences of an unexpected pregnancy.
Second, why does Gabriel reinforce his answer by appealing to Elizabeth?
Elizabeth's conception is presented as something humanly impossible:
"Your kinswoman Elizabeth in her old age has also conceived a son ... she who was called barren."
Gabriel then concludes:
"For with God nothing shall be impossible."
Elizabeth's case is relevant because her conception required divine intervention to overcome a natural impossibility.
Elizabeth is presented as both advanced in age and previously barren. Those are the obstacles that make her conception humanly impossible and therefore an appropriate example of God's power.
But if Mary's expectation was simply that She would later conceive through ordinary marital relations with Joseph, where is the analogous impossibility?
Conception through one's future husband is ordinarily possible and requires no appeal to God's power over impossibilities.
And even if it is argued that Mary had already inferred from prophecy or from Gabriel's words that this conception would occur miraculously, that still leaves the same question.
Elizabeth's obstacle is explicitly identified: advanced age and barrenness.
What is the corresponding obstacle in Mary's case that makes Gabriel's appeal to Elizabeth and his conclusion, "For with God nothing shall be impossible," a fitting response?
That is the narrative feature I am still trying to account for.
Even granting your proposed reason for Mary's question, I still do not see why Gabriel answers with a description of miraculous conception and then appeals to Elizabeth as evidence that God can overcome what is naturally impossible.