[wagglebee]

Mary giving birth to others would give a dubious status of Brother of God to the putative sibling;

Which would have created a de facto royalty that could have very possibly continued to this day with people claiming to be direct descendants of God's siblings.

[annalex]

would have created a de facto royalty

But of course. And conversely, note that when the scribes assault Christ's authority they point that he is one of many brethren:

55 Is not this the carpenter's son? Is not his mother called Mary, and his brethren James, and Joseph, and Simon, and Jude: 56 And his sisters, are they not all with us? Whence therefore hath he all these things?