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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 [betrothed]? 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?

The contention is that this is what is conveyed by "I know not man". Why St. Luke did not elaborate is a good question, but a similar elaboration is missing on the essentials of the Faith, such as the Trinity, -- the stone of stumbling of the early Church or the exact relationship between the works and the faith, which putatively caused the Reformation. However, the rough outline for the need of "josephine" marriage is known from tradition and is reflected in the Protoevangelium of James: that a temple virgin would have to be married off to a suitable man who would take care of her economic life in a patriarchic society without a carnal aspect normally associated with marriage. Since temple virgins were few, the scripture does not deal with regulating the aspects of such marriages.

8 [...] Mary was in the temple of the Lord as if she were a dove that dwelt there, and she received food from the hand of an angel. And when she was twelve years old there was held a council of the priests, saying: Behold, Mary has reached the age of twelve years in the temple of the Lord. What then shall we do with her, [lest] perchance she defile the sanctuary of the Lord? And they said to the high priest: Thou standest by the altar of the Lord; go in, and pray concerning her; and whatever the Lord shall manifest unto thee, that also will we do. And the high priest went in, taking the robe with the twelve bells into the holy of holies; and he prayed concerning her. And behold an angel of the Lord stood by him, saying unto him: Zacharias, Zacharias, go out and assemble the widowers of the people, and let them bring each his rod; and to whomsoever the Lord shall show a sign, his wife shall she be. And the heralds went out through all the circuit of Judaea, and the trumpet of the Lord sounded, and all ran.
9. And Joseph, throwing away his axe, went out to meet them; and when they had assembled, they went away to the high priest, taking with them their rods. And he, taking the rods of all of them, entered into the temple, and prayed; and having ended his prayer, he took the rods and came out, and gave them to them: but there was no sign in them, and Joseph took his rod last; and, behold, a dove came out of the rod, and flew upon Joseph's head. And the priest said to Joseph, Thou hast been chosen by lot to take into thy keeping the virgin of the Lord. But Joseph refused, saying: I have children, and I am an old man, and she is a young girl. I am afraid lest I become a laughing-stock to the sons of Israel. And the priest said to Joseph: Fear the Lord thy God, and remember what the Lord did to Dathan, and Abiram, and Korah; how the earth opened, and they were swallowed up on account of their contradiction. And now fear, O Joseph, lest the same things happen in thy house. And Joseph was afraid, and took her into his keeping.
The Protoevangelium of James

There is no claim that the Protoevangelium is inspired scripture and the authorship is in doubt. It is, however, an early Christian document that reflects the culture of at the latest the 2c, and it describes the marriage of St. Joseph and the reasons for it.