Look up the Greek.
I have not found an online translator that will translate "Protokos," can you suggest one? Aside from that, does it escape you that to translate from Aramaic into ancient Greek and then into English, the potential to misconstrue words? From the moment of His birth, Christ was "firstborn" that does not presume further children. And again I ask, if this was so obvious, why was it "overlooked" for more than fifteen hundred years? And to take it a step further, in what precise way would the Blessed Virgin's perpetual virginity impeed your anti-Catholic beliefs? You can deny the Immaculate Conception and Assumption without denying perpetual virginity, same with Papal Infallibility, etc.
I explained to you that "prototokos" is indeed firstborn, -- it is not a question of translation. It is a question of context, -- in a Jewish family it designates an obligation to God and a dynastic relationship, in this case to King David.