The pagans of old times had beautiful and talented young women who served in the temple. These women were called "temple virgins" and were held in very high esteem, like priestesses. On the other hand, Jews were (and are) very mindful of God's command: be fruitful and multiply. Having no children was considered shameful. (In some circles, it still is.)

How many times do we read in the Bible of childless women begging God to "remove my shame (by healing my infertility)"? There's even a commandment that a childless widow must marry her brother-in-law to "raise up children in his brother's name." Lot's daughters went so far as to get their father drunk and rape him. They saw such a heinous act as a lesser evil than remaining childless. Recall they had just survived a catastrophe that they believed to have wiped out all humanity but them. That's how serious the command to procreate is.

When Christianity was formed from misappropriated Judaism, it had to flex a bit to appeal to the pagans. Equating Mary with the familiar temple virgin priestesses would have been a logical way to do this. It would have fit right in with Paul's anti-sex stance.