Here’s a couple of interesting ways to look at this, besides all the other great responses here:
First of all, we believe Mary was conceived without sin, the perfect “New Ark” or tabernacle to house the growing body of our Lord in her womb.
Okay…let’s just suppose for the sake of the argument that she did have children with Joseph after the birth of Jesus. Knowing that every couple since Adam and Eve pass on Original Sin to their offspring, then what kind of children would Mary and Joseph have? Mary was free from Original Sin. Joseph was not. Would their children only inherit 1/2 of Original Sin? Does that mean they’d only sin half the time? What about *their *kids? Are they 1/4 without sin? What about 20 generations down the road. Doesn’t this give you a headache to think about what it would mean theologically?
Second, Mary has always been called the Spouse of the Holy Spirit. If Mary was also living with Joseph in a traditional husband-and-wife relationship and having children…well, wouldn’t you call it bigamy? Sure, this sort of thing went on back then, but it was not God’s plan for us. (After all, He didn’t create Adam and Eve and Susan and Gertrude and …)
Mary is already the spouse of another, the Third Person of the Holy Trinity. (I didn’t see any divorce decree issued anywhere, either.) Do you think that Joseph would have stepped in on the wife of another? His “job duties” were to be guardian and caretaker and to provide his leadership to the Holy Family.
Third, think about what would happen through the ages if Mary and Joseph had children, who would be half-siblings to Jesus. I’d bet my last dollar that sometime, somewhere, that someone would have “used” that connection for their own gain. (Can you say “nepotism?”) I can hear it now…“You have to appoint me to be _____ (fill in the blank)–emporer, king, president, high priest, head dog catcher, whatever…because my great-great-great-etc-etc-grandfather was the half brother of Jesus and that makes me better than you!” I really think there would unscrupulous people who would use a relationship like that, if it had happened. In fact, it is known that royal genealogists in England often “enhanced” some of the birthlines of the ruling families and would claim an ancestry back to the family of Jesus in an effort to make it look like these people had more of a right to sit on the throne than some other guy. Anyone who has seen the “Anna, cousin of Mary the Blessed Mother” family lines knows what I’m talking about. I’ve seen people try to connect their family lines to a relative of Mary before, but I’ve never seen anyone with the brass to claim they were a half-descendant of Jesus. That would really be crossing the line. Even those old royal genealogists from the 12th century didn’t go quite that far.
There’s just no way that Mary had other children, and Jesus had no biological half-siblings. Mary was a unique person with a unique, never-to-be-repeated Grace, with a special and unique relationship to the Holy Trinity–daughter of the Father, mother of the Son, spouse of the Holy Spirit. Jesus didn’t have biological siblings–so that no one could ever say, “Well, this guy is your bio-brother, and that woman, your bio-sister–and they must be somehow better than this other one over here who is not your bio-sibling.” (We are all his siblings by adoption through our baptism, anyway.)
Anyway, that’s just a different way of looking at it.