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.
See 50302. The point is irrelevant. The angel did not say that Mary would conceive that day, or next week. Then angel only said that she was "going to."
Think about it.
SD