It could be joked that during the season of Advent Protestant are forced to recognize the importance of Mary. Against the backdrop of the Roman Catholic Church's many dogmas about Mary, it makes sense that Protestants would de-emphasize her. However, in some cases, that de-emphasis has led to poor conclusions.
One such conclusion is to toss out the Perpetual Virginity of Mary. Without making a case for such a doctrine (and certainly never accepting it as a dogma of the church), John Calvin aptly avoided the controversy in many of his commentaries. What follows are mere quotations from John Calvin on passages pertaining to the Perpetual Virginity of Mary.
With respect to Matthew 13:55 (and Mark 6) Calvin wrote,
It was, we are aware, by the wonderful purpose of God, that Christ remained in private life till he was thirty years of age. Most improperly and unjustly, therefore, were the inhabitants of Nazareth offended on this account; for they ought rather to have received him with reverence, as one who had suddenly come down from heaven. They see God working in Christ, and intentionally turn away their eyes from this sight, to behold Joseph, and Mary, and all his relatives; thus interposing a veil to shut out the clearest light. The word brothers, we have formerly mentioned, is employed, agreeably to the Hebrew idiom, to denote any relatives whatever; and, accordingly, Helvidius displayed excessive ignorance in concluding that Mary must have had many sons, because Christ’s brothers are sometimes mentioned.
When addressing 1 Corinthians 9:5 Calvin wrote,
In the first place, he brings forward the Apostles He then adds, “Nay, even the brethren of the Lord themselves also make use of it without hesitation — nay more, Peter himself, to whom the first place is assigned by consent of all, allows himself the same liberty.” By the brethren of the Lord, he means John and James, who were accounted pillars, as he states elsewhere. (Galatians 2:9.) And, agreeably to what is customary in Scripture, he gives the name of brethren to those who were connected with Him by relationship.
Regarding Galatians 1:19 Calvin wrote,
Who this James was, deserves inquiry. Almost all the ancients are agreed that he was one of the disciples, whose surname was “Oblias” and “The Just,” and that he presided over the church at Jerusalem. Yet others think that he was the son of Joseph by another wife, and others (which is more probable) that he was the cousin of Christ by the mother’s side: but as he is here mentioned among the apostles, I do not hold that opinion. Nor is there any force in the defense offered by Jerome, that the word Apostle is sometimes applied to others besides the twelve; for the subject under consideration is the highest rank of apostleship, and we shall presently see that he was considered one of the chief pillars (Galatians 2:9). It appears to me, therefore, far more probable, that the person of whom he is speaking is the son of Alpheus.
And finally in Matthew 1:25 Calvin stated,
This passage afforded the pretext for great disturbances, which were introduced into the Church, at a former period, by Helvidius. The inference he drew from it was, that Mary remained a virgin no longer than till her first birth, and that afterwards she had other children by her husband. Jerome, on the other hand, earnestly and copiously defended Mary’s perpetual virginity. Let us rest satisfied with this, that no just and well-grounded inference can be drawn from these words of the Evangelist, as to what took place after the birth of Christ. He is called first-born; but it is for the sole purpose of informing us that he was born of a virgin. It is said that Joseph knew her not till she had brought forth her first-born son: but this is limited to that very time. What took place afterwards, the historian does not inform us. Such is well known to have been the practice of the inspired writers. Certainly, no man will ever raise a question on this subject, except from curiosity; and no man will obstinately keep up the argument, except from an extreme fondness for disputation.
One is certainly allowed to disagree with the analysis of Calvin. But perhaps it would be wise to acknowledge his warning about those with "an extreme fondness for disputation."