primarily because there are several places in scripture that lend very strong credence that this was not the case, and that Mary and Joseph were involved in a normal marriage which included intimacy.
Again to be clear, belief regarding Semper Virgo is not an article of faith, but this is not how Luther viewed it.
Luther accepts the view that brothers and sisters refers to cousins. He sides with Jerome against Helvidius.
“Helvidius, that fool, was also willing to credit Mary with more sons after Christ’s birth because of the words of the Evangelist: ‘And [Joseph] knew her not till she had brought forth her first-born Son’ (Matt. 1:25). This had to be understood, as he thought, as though she had more sons after the first-born Son.
How stupid he was! He received a fitting answer from Jerome.”
It is sometimes offered that “first born” implies there were more. Chemnitz, consistent with the Church Catholic, refutes this.
“Concerning the Firstborn the answer is easy. For in the law, when they are commanded to offer the firstborn to the Lord, the sense is not that one should wait until another one is born after the first one. Rather, he is called firstborn not only after whom others are born, but also before whom none is born, even if he should be the only-born; that is, even if he should afterward have no other brothers, he is still called firstborn.”
In other words, it confirms the past without predicting the future.
Finally, Walther claims the teaching is beyond question.
At the Milwaukee Colloquium between representatives of the Missouri and Iowa Synods, the following exchange took place:
Grossmann (Iowa): “When you subscribed to the Confessions, were you aware of the fact that they declared the permanent virginity of Mary?”
Walther (Missouri): “Yes, I can say so in the presence of God.”
Grossmann: “Do you still believe this to be
true doctrine?”
Walther: “Yes, I can say so in the presence of God.”
Grossmann: “What are your reasons for considering this a true presentation?”
Walther: “Pardon me, but you have no right to ask this question.”
(quoted from Beyer, Colloquium of Milwaukee, p. 43 sq., in J. L. Neve, A Brief History of the Lutheran Church in America, 1916 edition, p. 289)
Walther held quia subscription to the confessions.