You are perhaps not aware of the genesis (so to speak) of the term “first born son”. Actually, you need to go to the book of Exodus. God required of His chosen people that they redeem the first born male, i.e., the one who opened the womb.
Now let’s take a comparison of four average Jewish women.
Woman one gives birth to four children; two daughters and then two sons. None of her sons is a ‘first born son’. Her first daughter is not a ‘first born son’ either but is the ‘first child born’.
Woman two gives birth to three children, a son, a daughter, and a son. The first child is ‘a first born son’. The third child is a son, but not a ‘first born’ son.
Woman three gives birth to one child, a son. This child, by the Jewish law as set forth in Exodus, is the first born son who opens the womb, and is called so. . .even though the woman does not have any further children.
Woman four gives birth to one child, a daughter. While this child ‘opens the womb’, as a female she is not the ‘first born male’ of the law.
So the Virgin Mary was and is in the same position as woman three, above. She had ONE child, Jesus, who as a male child who opened the womb as per Jewish law was entitled the first born son.
And just try, please, to tell me that woman #3’s situation did not exist and that her one and only male child was not called the ‘firstborn son’.