Martin Luther: “It is an artcle of faith that Mary is the Mother of the Lord and still a virgin…Christ, we believe, came forth from a womb left perfectly intact.” (Works of Luther, V. 11, pp319-320; V. 6, p 510) See also this article.
John Calvin's Commentary on Matthew 1:25: "And knew her not 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."
John Wesley: "I believe that [Christ] was... born of the blessed Virgin Mary, who, as well after as before she brought Him forth, continued a pure and unspotted virgin."
Letter to a Roman Catholic, July 18, 1749.