Oh; I'd say about the same way you do...
Then you have to go by other reasonable evidence. I submit that when Hannah, Sarah, and Samson's Mom were told of their upcoming miraculous pregnancies, they all assumed that it was by their respective husbands that they'd get pregnant.
Mary doesn't assume that.
She's like that hypothetical bride-to-be at the bridal shower saying, astonished, "Babies? But how shall this be, since I know not man?" Either this bride-to-be doesn't know that the usual baby-making union with the husband is usually expected of the usual wife, OR --- the only other possibility --- she in no way expects to the the usual wife. There must be a reason why.