It suggest no such thing. what it means is that she was a virgin, not will always be a virgin.
"normally, a woman engaged to be married does not react in utter surprise when she is told that she will bear a child."
They do when the announcement says they will bear a child and there's been no sex! It doesn't matter if they're engaged. If there's been no sex the woman's going to be quite disturbed. Why I even remember that an angel visted Joseph to calm him down with an explanation.
The announcement refers to the future and the marriage, and presumably its consummation are in the future. The natural dialog would have gone like this:
- You will be with Child and He will be called the son of the Most High
- Great! I can't wait to tell my fiancee Joseph!
That's just your interpretation.
-A8