At Jesus' time when he appeared to be just another man--albeit and unusual one--that she did not give birth to other children would have been a shocking innovation. If she hadn't, and being so fundamental to doctrine, the Scriptures would have shown that she hadn't.
But, let's face it--it doesn't. To the contrary, it clearly says that she did, and other Scripture supports the idea the the general epistle of James (Iakob) and the letter of Jude were written by two of His brothers.
How much more plain can it be?......
God gave his ONLY begotten Son.
Joseph ‘knew’ not his wife until she had brought forth her FIRSTBORN son.
The argument that only older brothers would look down on Jesus is a weak one.
Extreme example: Children came out to mock Elisha ( which worked out pretty bad for the kids.)
Moses was 80 yrs old when he led Israel out of Egypt. A whole lot of folks didn’t have the respect they should have had for him. Of the 12 spies, only Caleb and Joshua showed respect to Moses (who was the prophet of God).
Samuel’s sons were not faithful like their father.
David had a son that sought to take his kingdom and his life.
Rehoboam and his young advisors wouldn’t listen to the older experienced men.
OTOH, a stated,deliberate, lifelong vowed virginity may well have been shocking. All the more reason to think that all those Christians who proclaimed Mary as Ever-Virgin, Asian, European and African, East and West, did not just make this up.
Like the doctrine of the Body and Blood of the Lord in the Eucharist, it's exactly the kind of radically disturbing, different thing they would have automatically rejected, unless they were convinced --- all of them --- that it were true.