Thanks for the response Tim. I can see it just as easily from the other side though.
For example, my niece is being raised as a good, God fearing girl. She also lives in a very conservative community. I promise you, if I, or someone else said to her right now, Rebecca, you will concieve a child, her response would be how

I’m not maried
This would not, and does not mean, she has no intention of ever getting married. She will at some point, I’m sure. But her response would reflect her present position and her understanding of right relations - children are to be born within wedlock.
If someone said to her Rebecca one day you will meet a nice young man, get married, buy a house, and have a child, she’d probably say way to go !!! It’s because the statement she would have a child came in the worng order of things.
Similarily, I can see Mary’s response in the same light. On hearing the news she will conceive a child, her response is, in essence, how, I’m not married. There is an immediacy implied in the Angels words, which may have thrown Mary as the marriage may have been some way off. We don’t know. We’re not told.
It does not necessarily preclude the fact she will get married, and have normal marital relations.
Like I said, I’m open to correction and rethinking.