A virgin birth story was the only excuse they could come up with to side step the born out of wedlock scandal.
Illegitimacy was taboo.
It is still a death sentence for some women in the region today.
Later the "miracle" played into the hands of religious leaders as "proof" of god's special power.

If you think about it...
As a result of the immaculate conception doctrine teachings like "In heaven as it is on Earth" and "In god's image" applies to everything about god except for fucking.
how sad.