Luk 2:7 And she gave birth to her firstborn son; and she wrapped Him in cloths, and laid Him in a manger, because there was no room for them in the inn.
Implying our Lord Jesus was Mary's "first" born and there were others. I believe I counted almost a dozen verses but it doesn't matter. People will choose to believe what they want.
Of course, but "firstborn" is indication of status, and does not imply that there was also second-born. You are, of course, correct that if one wants to read the scripture the way Full Court does, or the way Don Brown does, one can, if he disregards the Tradition of the Church.