Just curious: why "firstborn son" rather than just "son" or "child" if she had no other children later?
There is an emphasis upon “only son” wherever it applies in numerous passages within the Bible, in both Old and New Testaments, not only in the instance of the only begotten Son of God, so your question has merit.
Firstborn children are special under the Mosaic Law. Exodus 13:2 reads (God speaking), "Consecrate to Me all the firstborn, whatever opens the womb among the children of Israel, both of man and beast; it is Mine."
Mary had many other children: Matthew 12:46 While Jesus was still talking to the crowd, his mother and brothers stood outside, wanting to speak to him. Acts 1:14 They all joined together constantly in prayer, along with the women and Mary the mother of Jesus, and with his brothers. Mark 6:3
This is the builder, the son of Mary, and the brother of James, Joseph, Judas, and Simon, isn’t it? His sisters are here with us, aren’t they?” And they were offended by him. The meaning is clear: For neither did his brethren believe in him. At first they might take to him, and embrace him as the Messiah, and expect he would set up a temporal kingdom; in which they might hope, on account of their relation to him, according to the flesh to enjoy great honors and privileges; but finding that he was not inclined to anything of that nature, and talked in a quite different way, they grew sick of him, and rejected him, as the Messiah. The Bible mentions at least 6 children. Amen!
“firstborn son”. Exactly.
Also, “And he knew her not till”.
Since both Mary and Joseph knew that she was a virgin, why would that phrase even be there in the scriptures? KNEW HER NOT.......TILL?
And the ‘Mary born sinless’ invention; and her mother too?
Mary passed none of her blood into her child. The mother and child’s circulatory systems are completely separate, from conception through childbirth.