You are either ignorant of the meaning of First-born, or you are being dishonest. Luke knew what 'First-born' meant. The word First-born does not refer to the first child out of the womb. It refers to to the first male born.
Do you understand that?
And Luke says this explanation exactly where? Luke already said what about Mary? Really now, Mary was a virgin, divinely conceived, and Christ was her first born. Everybody was already told it would be God with us, (male) even way back to the prophet Isaiah.
What Luke did not say that others pretend is that Christ was the ONLY child that Mary birthed. Comprehend?????