On p. 284 of his book, Catholicism and Fundamentalism: The Attack on "Romanism" by "Bible Christians," Karl Keating wrote the following in an attempt to defend the dogma of Mary's perpetual virginity:
In the story of his being found in the Temple, Jesus, at age twelve, is mentioned as evidently the only Son of Mary (Lk 2:41-51); there is no hint of other children in the family. The people of Nazareth, where he grew up, refer to him as "the Son of Mary" (Mk 6:3), not as "a son of Mary". The Greek expression implies he is her only son. In fact, others in the Gospels are never referred to as Mary's sons, not even when they are called Jesus' "brethren". If they were in fact her sons, this would be strange usage.
Jesus being called ὁ υἱὸς τῆς Μαρίας (“the son of Mary”) in Mark 6:3 does not mean that Jesus was Mary’s only child; instead, it was an insult and insinuation that Jesus, unlike his other brothers and sisters, was born out of wedlock. As Jeni Broberg and Richard Neitzel Holzapfel wrote about this expression:
To describe a Jewish man as the “son of” his mother, even when the father was deceased, was an insult because Jews were customarily known by their father’s name (such as “son of Joseph”). This may be an insinuation of Jesus’ illegitimacy by local townspeople. On another occasion when Jesus talked about his Father, the Jews replied, “We be not born of fornication” (John 8:41). Here, the Jews may be turning to a well-known argument against Jesus. He has been talking about His Father in Heaven and about their father, but rumors about his own birth questioned whether Jesus was really the son of Joseph. The Jews may be saying in effect, “We were not born of fornication [but you were].” (Jeni Broberg Holzapfel and Richard Neitzel Holzapfel, Sisters at the Well: Women and the Life and Teachings of Jesus [Salt Lake City: Bookcraft, 1993], 43)
Keating reads too much into the use of the definite article (ὁ). If one wishes to read something singularly unique about Jesus in the use of ὁ, it would be that, as he would have been born earlier than expected (he was conceived before Mary and Joseph "came together" [Matt 1:18]), he, and not his brothers and sisters, would have been open to (seemingly, plausible) challenge of being illegitimate. It is not evidence for Mary being a perpetual virgin as Keating and some other Catholic apologists believe it to be.
For a full discussion of Mary's perpetual virginity examined in light of the Bible and the earliest patristic witnesses, including the arguments Keating uses in his book, see chapter 4: "The Perpetual Virginity of Mary" in my book, Behold the Mother of My Lord: Towards a Mormon Mariology (2017)