The reason why the woman that
carried the Christ-Child in the womb had to be a virgin
was because, first, it was prophesied in the OT
to be that way,
"A virgin shall conceive"
because God, Who was Christ in the flesh,
could not be associated with the "original sin"
that took place in the Garden of Eden with the Serpent,
WHO WAS AT THAT TIME, AN UPRIGHT BEAST, SUBTLE,
AND COULD TALK, and who "beguiled" Eve.
After that he was cursed and his legs came off, and he
became a crawling serpent...the missing link.
That's why the Holy Ghost Ghost overshadowed Mary
before she was married to Joseph.