Many of the answers here in this parallel question appeal (as OP seems to do) to the idea that Mary's question to Gabriel, "How can this be, since I know not a man?' only makes sense if she was already committed to remaining perpetually virgin. After all, the argument goes, she knew she was engaged, she knew consummation was coming, and she must have known babies come from consummation so the question makes no sense unless she also knew she had vowed never to 'know' a man.
Here is the link from which I am summarizing below.
But there is a time element that is not directly addressed in Luke's discourse which renders the above argument moot. A first century Jewish marriage happened in 3 distinct stages.
1) There is a betrothal (ketubbah) which is predominantly a legal contract (often between parents of the espoused). The betrothed couple has not yet consummated the marriage (the marriage bed) but the marriage is fully engaged at this point; thus, Joseph's thought to divorce Mary quietly makes sense because, although not yet consummated, the marriage was already valid in the betrothal.
2) Consummation (chuppah) occurs when the groom is able to fulfill the financial aspect of the betrothal contract with the father of the bride. He comes to the bride's house at the prearranged time where the bride is waiting with her maidens for the groom and his entourage. Consummation (sexual union) takes place in the bride's home while the wedding party waits (eww!). It may take up to 7 years between betrothal and consummation, depending upon the contractual obligations of the betrothal and the financial means of the groom.
3) The wedding feast (ala John 2) follows the consummation and the trek to the home of the groom.
Given the complete lack of information in Scripture regarding the specifics of the betrothal agreement including but not limited to; a) the exact date the betrothal contract was initiated, b) the agreed upon financial requirements placed upon the groom, and c) the expected time lapse between contract and consummation, it is not so hard to understand Mary's question.
Gabriel's pronouncement appears to lack a delay in the action foretold. He does not say, for instance, "At this time next year.", as was told to Abraham in Gen. 18. I do not know if the Greek grammar of Luke 1:31 indicates timing of the future fulfillment of this prophesy but it reads as more immediate than not. "The Lord IS with you.", and, "You HAVE found favor with God.", seem to indicate that some prerequisites have already been met.
Mary may well have understood the immediacy of the prophesy (as we who see it from this end of history know that it was) and known that their contracted consummation date was years in the future, as was common in her time and culture. Therefore her question to Gabriel, "How can this be?", might be paraphrased as, "How can I conceive soon, as you say, since I am currently a virgin and there are still 3 years until my marriage to Joseph is consummated?"
This understanding does nothing to diminish the character of Mary who had already found favor with the Lord and who, upon hearing a promise of the 'impossible', leaned into the promise with humility and faith; the faith of Abraham you might say. It is, however, a culturally plausible and scripturally sound alternative to the notion that the only logical ground for Mary's question is to assume a vow of perpetual virginity.
