Yes, but there's message and then the messenger. Would Mary and/or medieval theologians conceived (ha ha) of the idea that Mary could have been impregnated without some sort of inseminator? It's not that far-fetched in a world of ultrasound and MRI's and x-rays, but then?

If Mary were impregnanted by said omnipotent God, then an intermediary may have been necessary to make it real and comprehensible for HER, and not necessarily God's limitation. Even is Mary is physically untouched, the angel acts as an intermediary.

Finally -- and you'll have to forgive me because I don't remember the scientific biological specifics -- it IS theoretically possible that a woman can impregnate herself, although it would be very, very rare. Such an event, with or without divine intervention, would be a miracle and would NOT require a male parent. As I said, I don't recall all the particulars, but I remember it requiring an impressive number of unlikely events to converge.