Did you read what I posted about the use of firstborn? It can be used in a way to imply "most excellent" without necessitating that a secondborn son exists. (Ex 22:29, Ps. 105:36). an only child for example is still a firstborn son. The text says firstborn, not eldest, which would be a huge difference.
Again, it's adiaphora...I'm just shocked at the level of resistance to a doctrine which has been taught in the Lutheran Church. It's as if Semper Virgo is the newer position on the issue.
SD VIII 24
On account of this personal union and communion of the natures, Mary, the most blessed Virgin, bore not a mere man, but, as the angel [Gabriel] testifies, such a man as is truly the Son of the most high God, who showed His divine majesty even in His mother's womb, inasmuch as He was born of a virgin, with her virginity inviolate. Therefore she is truly the mother of God, and nevertheless remained a virgin.
