Mary was a normal girl, betrothed to a normal guy, and they expected to have sex as a married couple, but hadn't yet because they weren't yet in the consummation phase. The interval between betrothal and consummation was up to 12 months. So they were somewhere in that waiting period. She hadn't had sex yet ("I know not a man") but expected to, so when Gabriel said she's going to get pregnant, her natural response was (because she assumed this would happen before her marriage was consummated), how is this possible since I haven't had sex? And Gabriel affirms this by saying, look Mary, God is the God of the miraculous. Joseph didn't know this, so when she got pregnant he was like, WHAT? And he thought she committed adultery, which is why he wanted to divorce her until the angel said hold off there brother. He kept her a virgin until Jesus was born (Matt 1:25). Later she and Joseph had other children. (Matt. 12:46; 13:55; Mark 6:3; John 2:12; 7:3, 5, 10; Acts 1:14; 1 Cor. 9:5; Gal. 1:19).
I appreciate that you've finally laid out your entire interpretation in one place. That makes it easier to identify where our disagreement actually lies.
However, I don't think you've answered the central difficulty of the challenge. Rather, you've offered a possible scenario that depends on several assumptions that Luke himself never provides.
You propose that Mary expected a normal marriage with Joseph, including sexual relations; that She understood Gabriel's announcement to mean She would conceive before their marriage was consummated; and that this explains why She asked, "How will this occur, since I know not man?"
But notice what your explanation requires us to assume. You assume Mary understood Gabriel to be announcing an immediate conception, or at least one that would occur before the consummation of her marriage. Yet Gabriel never says that. He says, "You will conceive and bear a Son."
You suggest that perhaps it was Gabriel's tone or the suddenness of the announcement that led Mary to that conclusion. But neither of those explanations is actually supplied by Luke. They are possibilities you are introducing to make your interpretation work. And this brings us back to the question: Why should we give greater interpretive weight to an assumed tone and an assumed timetable than to the actual words Mary speaks?
Mary doesn't ask when She will conceive. She asks HOW She will conceive, grounding Her question in the fact that She does not know a man.
You also say that Mary would naturally have expected sexual relations because She was betrothed to Joseph and intended to have a normal marriage. I understand the cultural argument. Ordinarily, a betrothed woman would expect her marriage to proceed to consummation. But that general cultural expectation does not explain why Mary responds to Gabriel's announcement as She does.
If She simply understood Gabriel to be announcing that She would eventually have a child through the ordinary marital relations She anticipated with Joseph, what exactly was the difficulty? You say the difficulty was that She thought conception would occur before those relations. But that is precisely the assumption that needs to be established, not simply repeated.
Consider Gabriel's actual response. He doesn't merely reassure Mary that She will eventually conceive through Joseph. He explains: "The Holy Spirit will come upon you, and the power of the Most High will overshadow you." Then he introduces Elizabeth: "Behold, your kinswoman Elizabeth in her old age has also conceived a son..." And concludes: "For with God nothing will be impossible."
Elizabeth's natural obstacle is explicit: she is barren and advanced in age. But what is Mary's corresponding obstacle? Under your interpretation, Mary is young, fertile, betrothed, and anticipating ordinary marital relations with Joseph.
You identify Her virginity as the obstacle, but then explain that obstacle away by saying it was merely temporary and that She expected it to end through ordinary marital relations. That still leaves the question: Why does Gabriel answer Her by describing a miraculous conception through the Holy Spirit and invoking Elizabeth's otherwise impossible pregnancy?
The comparison seems much more natural if Mary's difficulty is not simply that She has not yet reached the customary time for marital relations, but that the conception being announced will occur without a man.
I'm not claiming that Luke 1:34, taken in isolation, mathematically proves every aspect of Mary's lifelong virginity. I'm pointing out that the passage presents a genuine interpretive difficulty for the position you are defending.
Now, regarding the rest of your response, you appeal to Matthew 1:25, saying Joseph kept Mary a virgin until She gave birth, and then conclude that they subsequently had relations. But the word "until" does not necessarily establish a change of circumstances after the specified event. Scripture uses similar constructions without implying that the situation subsequently changed. Matthew's statement establishes that Joseph did not have relations with Mary before Jesus' birth. It does not explicitly tell us what happened afterward. I recommend checking out my thread
Does "Until" Mean What You Think It Means? (Matthew 1:25).
Likewise, the references to Jesus' brothers and sisters establish that these relatives existed, but they do not, by themselves, establish that they were Mary's biological children. That is another interpretive conclusion, not an explicit statement of the passages. I recommend checking out my thread
Jesus’s Adelphoi: Scripture and Early Christian Testimony.