I have "lifted" this from BeggarsAll - because I thought it an interesting comment and my "response" doesn't really "fit" with Swan's sentiments. Plus, if I'm going to respectfully disagree with an alleged statement by a Catholic Saint, I'll do so over here! ;-) Being a former Lutheran myself and I would like to look into this particular topic more. Here is what is found on BeggarsAll:============================
If Luther is willing to accept nothing except what is plainly set down in Scripture, why does he believe in the perpetual virginity of Mary? There is nothing to prove this in Scripture and Helvidius actually took it upon himself to prove the contrary, relying on no other authority than that of Scripture.
Thomas More, "An Answer to Martin Luther," The Essential Thomas More (Canada: Mentor-Omega Books, 1967), p. 112.
Now, while I would go so far as to say that the perpetual virginity of the Blessed Virgin is not explicitly taught in Scripture and is anchored in Sacred Tradition - it is at least implied by Scripture. The following comes from the Scripture Catholic site:
Mark 6:3 - Jesus was always referred to as "the" son of Mary, not "a" son of Mary. Also "brothers" could have theoretically been Joseph's children from a former marriage that was dissolved by death. However, it is most likely, perhaps most certainly, that Joseph was a virgin, just as were Jesus and Mary. As such, they embodied the true Holy Family, fully consecrated to God.
Luke 1:31,34 - the angel tells Mary that you "will" conceive (using the future tense). Mary responds by saying, "How shall this be?" Mary's response demonstrates that she had taken a vow of lifelong virginity by having no intention to have relations with a man. If Mary did not take such a vow of lifelong virginity, her question would make no sense at all (for we can assume she knew how a child is conceived). She was a consecrated Temple virgin as was an acceptable custom of the times.
Luke 2:41-51 - in searching for Jesus and finding Him in the temple, there is never any mention of other siblings.
John 7:3-4; Mark 3:21 - we see that younger "brothers" were advising Jesus. But this would have been extremely disrespectful for devout Jews if these were Jesus' biological brothers.
John 19:26-27 - it would have been unthinkable for Jesus to commit the care of his mother to a friend if he had brothers.