Everything about this subject is personal interpretation, because the Bible does not clarify it.
Incorrect. Scripture makes no mention of the Blessed Virgin Mary having any other children. Scripture does indeed clarify who the biological parents of the "brothers" of Christ are. In addition, Scripture calls Jesus "the" son of Mary never "a" son of Mary and Scripture never calls these "brothers" the sons of the Blessed Virgin Mary.
Linguistic literalists need to heed the words of St. Peter:
"As also in all his epistles, speaking in them of these things; in which are certain things hard to be understood, which the unlearned and unstable wrest, as they do also the other scriptures, to their own destruction." 2 Peter 3:16