That isn't what Luther wrote. As per Jaroslav Pelikan (who is not a member of the CARM forums due to being a lot smarter than all of us here and also being dead), the Latin text of the Smalcald Articles is a translation of the original German text. The original German text
did not contain the notion of "Mary, pure, holy, and ever-Virgin" (
ex Maria pura, sancta, Semper Virgine.) See his comments in his book,
Mary Through the Centuries (New Haven: Yale University Press, 1996), 158-159.
Some years back I interacted with one of Rome's defenders, who, in essence thought the German versions of the Smalcald Articles were engaging in a Marian
cover-up. Here we see that the actual historical distortion is attributing the Latin text to Luther's German. The question though is why did the Latin text use "
ex Maria pura, sancta, Semper Virgine"?
Why was the Latin was translated “Mary, pure, holy, and Ever-Virgin”? Perhaps the the translator used the phrase as a force of habit (as it was a common phrase). Perhaps the translator believed in Mary’s perpetual virginity and Immaculate Conception. It could also be that the Smalcald princes thought the phrase would gain more in a hearing with the Papacy? It’s also possible that the sense of “pure” was only to indicate that Mary was a “
real creature or truly human” [See: Heiko Oberman,
The Impact of the Reformation (Michigan: WB Eerdmans Publishing Company, 1994) 241.] which was a sense in which this word was used in the 16th century.
Details of the Latin translation / translator can be found
here (see the discussion starting on page 332). The Latin translator "
fell into disrepute" as per "
evinced Roman Catholic leanings."
And now you know... the rest of the story!
Edited to add: The thrust of my comments is not to deny Luther's adherence to Mary's perpetual virginity. The above should be read with the emphasis on the notion of the immaculate conception: a Roman deviation which Luther did ascribe to early in his career, but later rejected.