This post is courtesy of Perry Robinson of energetic procession: https://energeticprocession.wordpress.com/2009/11/13/sometimes-you-feel-like-a-nut-francis-turretin-on-the-perpetual-virginity-of-mary/.
The perpetual virginity of Mary often comes up in Protestant and Catholic arguments. When I was Reformed, I didn’t think much of it. I didn’t think it ought to be a church dividing issue since plenty of the Reformers and post-Reformation theologians adhered to it and defended it. And I didn’t at the time see its significance one way or other, though no I think reflection on the perpetual virginity of Jesus will show that it is.) It was also a teaching that was held and judged to be correct or at least permissible, by plenty of “secondary” authorities. If Sola Scriptura entails following secondary authorities and eschewing the supposedly more Anabaptist take of solo Scriptura, this seemed like such a case. And of course, plenty of Protestants hold to it today, not the least of which are the Lutherans.
Francis Turretin (1623-1687) no small name among Reformed authors also favored the doctrine. Here is his summary defense of the teaching.
“This is not expressly declared in Scripture, but is yet piously believed with human faith from the consent of the ancient church. Thus it is probable that the womb in which our Savior received the auspices of life (whence he entered into this world, as from a temple) was so consecrated and sanctified by so great a guest that she always remained untouched by man; nor did Joseph ever cohabit with her.
Hence Helvidius and the Antidicomarianites (so-called because they were opponents of [antidikoi] Mary) are deservedly rebuked by the fathers for denying that Mary was always a virgin (aei Parthenon). They held that she cohabited with Joseph after delivery; yea, also bore children from him. As Augustine remarks, they rely on the shallowest arguments, i.e., because Christ is called the ‘firstborn’ of Mary (cf. De Haeresibus 56, 84 [PL 42.40, 46]). For as Jerome well remarks, she was so called because no one was begotten before him, not because there was another after him. Hence among lawyers: ‘He is the first whom no one precedes; he is last, whom no one follows.’ The Hebrews were accustomed to call the firstborn also only begotten; Israel is called ‘the first-born of God’ (Ex 4:22), although the only people chosen of God. Thus ‘the firstborn’ is said to be ‘holy unto God’ (Ex 13:2), who first opened the womb, whether others followed or not. Otherwise the firstborn would not have to be redeemed until after another offspring had been procreated (the law shows this to be false because it commands it to be redeemed a month after birth, Num. 18:16).
Not more solidly have they been able to elicit this from the fact that in the New Testament certain ones are called ‘the brothers of Christ.’ It is common in Scripture not only for one’s own and full brothers by nature to be designated by this name, but also blood relatives and cousins (as Abraham and Lot, Jacob and Laban). Thus James and Joses, Simon and Judas are called brothers of Christ (Mt. 13:55) by a relation of blood. For Mary (who is called their mother by Matthew and Mark) is called by John the sister of the Lord’s mother. However what is said in Jn. 7:5 that ‘neither did his brethren believe him’ must be understood of more remote blood relations.
Nor is it derived better from this-that Joseph is said ‘not to have known Mary till she had brought forth her firstborn son’ (Mt. 1:25). The particles ‘till” and ‘even unto’ are often referred only to the past, not to the future (i.e., they so connote the preceding time, concerning which there might be a doubt or which it was of the highest importance to know, as not to have a reference to the future-cf. Gen 28:15; Pss 122:2; 110:1; Mt.28:20, etc.). Thus is shown what was done by Joseph before the nativity of Christ (to wit, that he abstained from her); but it does not imply that he lived with her in any other way postpartum. When therefore she is said to have been found with child ‘before they came together’ (prin e synelthein autous), preceding copulation is denied, but not subsequent affirmed.
Although copulation had not taken place in that marriage, it did not cease to be true and ratified (although unconsummated) for not intercourse, but consent makes marriage. Therefore it was perfect as to form (to wit, undivided conjunction of life and unviolated faith, but not as to end (to wit, the procreation of children), although it was not deficient as to the raising of the offspring.”
Francis Turretin, Institutes of Elenctic Theology, vol. 2, 345-346.
FURTHER READING
THE REFORMERS ON MARY’S PERPETUAL VIRGINITY
REFORMERS ON THE FEAST OF THE ASSUMPTION
What the Early Church Believed: The Perpetual Virginity of Mary
Answering Islam Blog 
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