There were many wonder-pregnancies in the OT, as Mary would have known full well, because she was steeped in those stories. Hannah, Sarah, Samson's mother and the rest. And they were to understand that they would conceive by their husbands.
Now, Mary had a husband (Joseph). Why would she be astonished to think she'd get pregnant? Why wouldn't her first thought be that it would be in the ordinary way with her husband?
Now put your thinking cap on and think through this example. Say you're at a wedding shower and all the young women are talking excitedly about the upcoming wedding. They're looking at pictures, commenting in the cuteness of the happy couple, and one of the girls says, "Oh, Olivia, you are going to have such gorgeous children!"
And Olivia says, astonished and perplexed, "But how shall this happen to me, because I know not man?"
???
Huh?
Seriously?
That makes no sense.
Reasonably, once you've eliminated the impossible, whatever remains, however improbable, is the true explanation. Either this gal, betrothed wife of Mr. Right, doesn't know where babies come from (impossible!), or --- this is so improbable, but --- isn't she planning on having sexual relations with him?
As for Joseph divorcing Mary quietly, that doesn't serve as a counter-argument. That doesn't imply anything, except that he's perplexed: is he supposed to be the guardian of her bastard children as well? But he was kind and did not want her exposed to a public shaming. As it was, as soon as the angel provided the explanation about the Divine origin of this Child, he took her into his home as planned, and it was for all the world to assume that the child was Joseph's.
And my husband has AGREED to never ever try to seduce me into that nasty stuff I heard about in the Family Planning classes we took!
Sorry; but your 'EXPLANATION' needs some 'spalinin' of it's own; since it contracts the assertion YOU made about being a 'guardian'.
Mary’s statement wasn’t, “How shall this be because I never intend to have sex with any man?’
It was “How can this be (present tense) since I am not currently having sex with a man?”
It says NOTHING in the least about future intentions.