I have taken no position whatsoever on whether Mary's virginity was temporary or permanent. I've just tried to answer the questions the OP is asking.
Well, then perhaps I've misunderstood your aim.
As made clear in the opening post, the challenge is specifically directed at those who assert that Mary expected ordinary marital relations with Joseph and that Joseph and Mary later had marital relations after the birth of Jesus. In other words, it is directed at those who understand Mary's virginity to be temporary.
So when you attempted to answer the challenge, I naturally assumed you were defending that position.
Moreover, your explanation seems to presuppose that Mary expected future ordinary marital relations with Joseph. After all, your explanation is that Mary was puzzled because She understood conception would normally occur through those future relations and therefore interpreted Gabriel's announcement as referring to a conception that would occur beforehand.
If that's not the expectation you're appealing to, then I'm not sure what future expectation is supposed to explain Mary's question.
So while you may not be taking a position on what happened later in Mary's life, your explanation does seem to presuppose that Mary expected future ordinary marital relations with Joseph as the normal means of conception.
If you're not actually taking a position on whether Mary's virginity was temporary or permanent, then we're discussing a somewhat different question. The issue I raised is not whether
some hypothetical explanation can be imagined for Mary's question, but whether the explanation commonly offered by those who reject lifelong virginity adequately accounts for the details of the text.
So, just for clarification, and so I know how to address your answer to the challenge: do you believe Mary expected ordinary marital relations with Joseph in the future, or not? Your explanation appears to depend on that assumption.