Just to jump in:
- I trust you're not saying Calvin and Luther were "early" Christians.
- Josephus was an early Christian, and he said that James was the brother of Jesus.
But, to me, the real question is why this is even a concern - it's about Jesus, not his mother. Jesus is my Saviour, and I worship Him - He is the only begotten Son of God, born of a virgin, crucified for our sin, buried, and raised from the dead - period. Whether or not Mary remained a virgin after Christ's birth is of no relevance, unless you worship her!
I think this is less about Mary’s sex life than it is about the authority of the Roman Church. They are the original “slippery slopers”. Reject one doctrine and the whole edifice crumbles. I have more faith in the power of the Holy Spirit than that.
They know the Perpetual Virginity of Mary is one of the weakest doctrines so that means they have to defend it even more fiercely.
I suggest you might be drawing the wrong conclusions from scripture. Whether Jesus had any blood siblings is unclear, as far as I can make out from the excellent exposition made. It doesn't seem to say one way or the other. But rather than that proving the case either way, it would suggest to me that the writers of the Bible thought the whole issue wasn't all that important. The emphasis in the NT seems to be more on a spiritual brotherhood/sisterhood than blood ties.
Extra credit for you ...