I learned about the Luke and Matthew virgin birth narrative possibly being a cover story for a premarital pregnancy from a
Catholic scholar:
"Only Luke and Matthew give us the news that Mary was pregnant before she and Joseph were married.
It would be difficult to overstate the impact of a premarital pregnancy on a Jewish observant village such as Nazareth. A premarital pregnancy, if the father were not the intended husband, would bring great dishonor and shame on a woman’s family. According to the Torah, Mary’s relatives would have been entitled to take her out of her parental house and punish her through stoning.
Many scholars are prepared to accept that Mary’s premarital pregnancy must therefore be a historical event. Usually, when such potentially damaging stories appear in the Gospels, it means that the underlying oral or written tradition was simply too persistent or well known for the writer to ignore.
Both Luke and Matthew use the Hebrew Bible to frame the story in a theological context."
- Jean-Pierre Isboot, practicing Catholic and Biblical Scholar