LOL Another Scripture scholar. Let me get this straight. You are telling me, with a straight face, that the phrase "firstborn" means that there had to be more than one?
You might want to study the Old Testament and the peoples found within.
SD
Firstborn is a legal term indicating a special privilege or rank. Psalm 88:28 "And I will make him my firstborn, high above the kings of the earth." David is referred to as the firstborn even though he was Jesse's eighth son. Colossians 1:15 "Who is the image of the invisible God, the firstborn of every creature:" Many people were born prior to Jesus, St.Paul is indicating that Christ has primacy over all creatures. The use of the term "firstborn" in Matthew 1:25 does not prove that Mary had other children and if she had then Sacred Scripture would tell us who they are.