A third objection to the perpetual virginity
of Mary arises from the use of the word,
prototokos, translated "first-born" in Luke's
gospel.
Luke 2:7
(Mary) gave birth to her firstborn son (prototokos). She wrapped him in swaddling clothes and laid him in a manger ...
The Greek word prototokos is used of Christ as born of Mary and of Christ's relationship to His Father (Col 1:25). As the word does not imply other children of God the Father, neither does it imply other children of Mary. The term "first-born" was a legal term under the Mosaic Law (Ex 6:14) referring to the first male child born to Jewish parents regardless of any other
children following or not. Hence when Jesus is called the "first-born" of Mary it does not mean that there were second or third-born children.
Virgin Mary Directress