It didn't "develop"; it was always what was believed, and is the only possibility that is consistent with Mary's words to the angel, and Jesus' giving Mary to John on Calvary.
Jesus said over and over that the family of His Kingdom would be a spiritual one. At some point Jesus’ brothers didn’t believe in Him, and regardless of when they did, they did not follow Him all the way to the cross. But John did. And when someone says these were only Jesus’ cousins, who could also be called brothers, then if such kinsman were considered to be that close, then they should also have a duty to care for Mary as a mother.
It is not consistent with anything.
The Bible only states that Mary was a virgin at the time of her conception of Jesus at which time she was betrothed to Joseph.
The Bible does not state that Mary and Joseph practiced celibacy. The Bible does mention Jesus’s brothers and sisters.
It is therefore reasonable to assume that Mary and Joseph had loving marriage and shared physical intimacy. A gift from God not to be denied to anyone, certainly not Mary and Joseph. The Lord commands us to be fruitful and multiply. And Mary and joseph did just that.