Ok, I'll respond. She said "How can this be, I know not a man". Obviously speaking in present tense not future tense. Really this is silly.
Left your nose out of the grammar and think about the situation.
If someone told your wife a few months before your wedding that she was going to conceive a child, would your wife say "How is this possible?"
This response makes no sense whatsoever unless the person was not expecting to engage in sex in the future.
SD
What an ironic sentence!
If I was a virgin and someone told me I was going to have a child my first question would be "how can this be". Don't start the day off with foolishness.
If someone told your wife a few months before your wedding that she was going to conceive a child, would your wife say "How is this possible?"
You're sure that there were only months before the impending nuptials ?
Joseph and Mary were betrothed. The marriage could have been years off.