when she said “how can this be since I know not man”. I believe we can pretty logically argue that Mary meant this to be that she has not known man in the past… we can also logically conclude that she meant that she didn’t plan on having sex in the VERY near future. This would then open the possibility that she may have planned to have marital relations in the distant future…
As I believe the conception of Jesus happened at the annunciation or within a day or two of the annunciation. And Mary probably understood this also.
How do we know this?
The Catholic calendar celebrates the feast of the annunciation on March 25th… what happens exactly 9 months later?
This would lead us to conclude that the most likely explanation is that she conceived at or shortly after the angel visited her.
Otherwise we would have to conclude that the annunciation happened at least a year before she actually conceived
that Jesus was born premature… which in that day and time would be very dangerous and even then you can only get away with a month or two. Typical Jewish betrothals could last for quite a while and Mary may not have planned to actually wed Joseph for many months.