If she saw herself as a literal fulfillment of Isaiah 7 in the sense of virgin birth (not the only reading of that passage, as you probably know), her question would have been "How can this be because I am engaged to be married?"
If she saw herself as a mother of the Messiah born through natural means, she would not have any question at all, as she was on her way to be mariied and soon a mother.
The only way her response in v. 34 makes sense is if she did not intend to have carnal relations with Joseph.
But you are still building on what Scripture doesn't say ANYWHERE. If what you say is true, then why wouldn't she have said, "how can this be since I have pledged myself to God to be a virgin all my life?" Why would she allow herself to be betroathed? If she were dedicated to God alone, why would she become some man's wife and allow her attention to be swallowed up in her relationship with him?
As Paul says: 1 Corinthians 7:34
There is difference also between a wife and a virgin. The unmarried woman careth for the things of the Lord, that she may be holy both in body and in spirit: but she that is married careth for the things of the world, how she may please her husband
Of course, in this context virgin is better understood as unmarried woman since Mary was Joseph's espoused WIFE and still a virgin. But Jewish law and Christian instruction indicated that one shouldn't avoid sexual relations with their spouse except for short periods of time for specific spiritual purposes.
1 Corinthians 7:3-5
3Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband.
4The wife hath not power of her own body, but the husband: and likewise also the husband hath not power of his own body, but the wife.
5Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.