I apologize if this has been mentioned already but let’s think about this.
You have Mary, who has been told by God that she is going to bear the Son of GOD. She is going to be responsible for Him- the promised Messiah, the one who is to save the people from their sins. This Messiah is basically the One who will be the culmination of Jewish history --THE most important person in the entire universe, EVER. . . and He’s going to be born to this young woman in a small, unimportant town.
So what does she do? Hey, I have God to care for, she thinks, but I have a ‘life’ of my own too. (never mind for the moment that her statement “how shall this be because I know not a man” reflects a woman who never intended to consummate a marriage, because any woman who was already betrothed would have understood that message from Gabriel as a prophecy for her to have a child WITH Joseph). . .
SO, she bears the Son of God, a helpless baby. Herod is searching to destroy the child. She and Joseph have to flee to Egypt for at least a few years. What’s a woman to do? Why, in the midst of all this, have more babies. Of course. Never mind the fact that most women would have a large extended family to care for the baby already there, and Mary, ON THE RUN, in a FOREIGN COUNTRY, had no support system at all. Never mind that having another child would strain their already thin resources. Never mind that adding a child who was ‘half blood’ to GOD on the mother’s side but full blood for Joseph would mean inheritance issues and would further intensify the doubts of Jesus’ paternity and stain Mary’s name. Never mind that having a perfect child (Jesus) and then other ‘mortal’ children would involve the whole family in rivalries where the other children could never 'live up to" Jesus. Never mind that instead of having her complete attention on Jesus, Mary and Joseph would be focused on others whom they could not help but feel were ‘all theirs’. Never mind that childbearing was RISKY and with each pregnancy, Mary ran the risk of dying and leaving behind not just Jesus but all the other children.
No, Mary and Joseph were just like any sex-saturated people of today. Life was only about sex. . .and hey, life then, being so much shorter, and with a need for ‘hands to help out’, was all about having lots and lots of babies. Of course they couldn’t possibly set aside their ‘sexual selves’ and focus on the task of raising the Son of God only. Pftt. How HARD is it to raise God? Surely He’d understand that it was ENOUGH for Mary to just say 'yes", have the first pregnancy ‘special’, and then get on with real life. Let Jesus know from the start the real ‘facts of life’ --it’s all about Mary and Joseph getting physical, and hey, what better way for God to be raised than with a bunch of other people. Put Him in His place. The family could holler for Him to perform a miracle if needed, the rest of the time, let Him sit back and try not to get in their way.