John Calvin (1509-1564), the second most important Protestant “Reformer” after Martin Luther, and probably more historically influential than even Luther himself, believed in the perpetual virginity of Mary. Here are the usual proofs offered in favor of his beliefs:

• “Helvidius displayed excessive ignorance in concluding that Mary must have had many sons, because Christ’s ‘brothers’ are sometimes mentioned.” (Harmony of Matthew, Mark and Luke, on Matthew 13:55)
• “Under the word ‘brethren’ the Hebrews include all cousins and other relations, whatever may be the degree of affinity.” (Commentary on John 7:3)
• “The inference he [Helvidius] 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. ... No just and well-grounded inference can be drawn from these words … 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. ... What took place afterwards the historian does not inform us. ... No man will obstinately keep up the argument, except from an extreme fondness for disputation.”  (Harmony on Matthew 1:25)

Some Protestants I have interacted with were unconvinced by these excerpts, so I produced more from Calvin. In “Tome 46” of the Corpus Reformatorum is found “Sermon 22” on the Harmony of the Gospels (from 1562), dealing with Matthew 1:22-25. The sermon is in French and appears on pages 259-272. I posted the entire French text on my blog. Here are some key excerpts, from a translation by Gregory Fast:

Certainly, it is said that he did not know the Virgin until she gave birth to her first Son. By this, the Evangelist means to signify that Joseph did not take his wife to live with him, but in obedience to God and to discharge his duty towards him. It was not then to be carnal love …

But there were some crazy people who wanted to gather from this passage that the Virgin Mary had had other children than the Son of God, after Joseph had lived with her … 

[He] took no regard for himself because he was deprived of a woman. He could have married another, but he could not swear off the woman he had engaged. But better he leave his beloved rights and abstain from marriage (even though all the while he was married) …

We also have further evidence in his writings that Calvin interpreted Jesus’ “brothers” in Scripture as his cousins. In his Harmony of the Gospels, Calvin is commenting on Luke 8:19 (“And his mother and his brethren came to him”), and  casually mentions that the parallel passages (Matthew 12:46; Mark 3:31) of “the other two Evangelists ... represent Christ’s mother and cousins as having come ...”In another instance of Calvin interpreting a “brother of Jesus” as a cousin, we have his commentary on Galatians 1:19 (“But I saw none of the other apostles except James the Lord’s brother.”). He states that this James is “the son of Alpheus.” Footnote 35 in this edition of Calvin’s commentaries elaborates:

This is fully consistent with the opinion commonly held, that Alpheus or Cleopas was the husband of the sister of Mary, the mother of our Lord, and consequently that James, the son of Alpheus, was our Lord’s cousin-german.

Calvin in his writings habitually called Mary “the virgin” or “holy virgin” (as Calvin scholar T.H.L. Parker noted), which is further evidence of his belief in her perpetual virginity. Here are examples from his most famous and influential work, Institutes of the Christian Religion:

• II, 10:4 — “... the blessed Virgin ...” [footnote: “Beata Virgo.” French, “la Vierge Marie” — the Virgin Mary]
• II, 13:3 — “... being descended of the Virgin ... nourished to maturity in the Virgin’s womb. ... Matthew does not here describe the Virgin …”
• II, 13:4 — “… conceived miraculously in the Virgin’s womb ...”
• II, 14:1 — “… he made choice of the Virgin’s womb as a temple in which he might dwell.”
• II, 14:4 — “... the name of the Son of God is given to him who is born of a Virgin, and the Virgin herself is called the mother of our Lord” and “he was begotten in the womb of the Virgin by the Holy Spirit. ... We indeed acknowledge that the Mediator who was born of the Virgin is properly the Son of God.”
• II, 14:6 — “... he who was born of a Virgin …”
• II, 14:8 — “... he was conceived in the womb of the Virgin by the Holy Spirit …”

Likewise, in his Harmony of the Gospels, he referred to Mary as “the virgin” seven times in his commentary on Matthew (1:18-19, 22-23; 2:16; 5:6), and “the virgin” or “the holy virgin” 25 times in his commentary on Luke (1:26, 28, 30-32, 34-36, 38-39, 46, 48-49; 2:34-35, 48).This is not simply referring to the virgin birth. We don’t call women who are married now and sexually active, “virgins” their whole lives and thereafter. That would make no sense, since they ceased being virgins. It is as illogical as calling them “children” when they are adults, if they’re not lifetime eunuchs or celibates or virgins.