If she had had a girl, things would have gone very differently...
and its not as if God wandered around Bethlehem wondering if his first born was a boy or a girl.
So why did he have a boy?
Not such a stupid question, (although mostly hero myths do involve boys) some of the Greek female gods had a bit of backbone.
Well, according to Jung, in his studies of the rise and fall of religious sects in ancient Egypt, as one new religion was trying to take power from the current one.... the current power was (almost obviously) branded as the devil. It seems that this is a universal pattern, echoed a bit in our "democratic process".
The last religion that ruled (or political party) become demonised by the current religious power.
And so we come to the question again, why did Mary not have a girl? The answer may be found in the preceding religion. The preceding religion in much of the Middle East and Europe was the cult of the Goddess. To have had a girl would have reinforced that particular view point, and given power to the priestesses.
And so it came to pass that a totally male dominated religion was born and all 3 of the top jobs were given to men, viz. Father, Son and Holy Ghost, although I suspect the Holy Ghost has (inspite of Mary's impregnation) more female attributes than male.
This unholy holy alliance was reinforced in the following centuries and women were demonised, religiously.....and the world, well, European world, fell down into a dark depression where Dogma was wielded like a blunt instrument, and intellectual activity was basically looked down on. The knowledge of the natural world that women had was hidden, then forgotten.
Well, thats all old history, you might think, but this decidedly dodgy state of affairs continues in our culture and our laws due to the historical influence of the Church. I'm not saying anything really new here, I know, but....at Christmas its good to remember thats Mary's little boy has been a curse as well as a blessing.
Mary could have had a baby girl.