Literally no one here is saying Mary was not a virgin when Jesus was born. They are not questioning her virginal motherhood.
And exactly how does having relations after Jesus was born affect the "truth that he was both fully God and fully man"?
Hope this helps
Martin Luther
It is an article of faith that Mary is Mother of the Lord and still a virgin. … Christ, we believe, came forth from a womb left perfectly intact. (Weimer’s The Works of Luther, English translation by Pelikan, Concordia, St. Louis, v. 11, pp. 319-320; v. 6. p. 510.)
John Calvin
(On the Heretic Helvidius) 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, sec. 39 [Geneva, 1562], vol. 2 / From Calvin’s Commentaries, translated by William Pringle, Grand Rapids, Michigan: Eerdmans, 1949, p.215; on Matthew 13:55)
[On Matt 1:25:] 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. (Pringle, ibid., vol. I, p. 107)
Under the word “brethren” the Hebrews include all cousins and other relations, whatever may be the degree of affinity. (Pringle, ibid., vol. I, p. 283 / Commentary on John, [7:3])
John Wesley
‘I believe that He [Jesus] was made man, joining the human nature with the divine in one person; being conceived by the singular operation of the Holy Ghost, and 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’, The Works of Rev. John Wesley, vol 10, p. 81).
St Augustine, Sermons 186.1 (early 5th century):
“In being born of a Virgin who chose to remain a Virgin even before she knew who was to be born of her, Christ wanted to approve virginity rather than to impose it. And he wanted virginity to be of free choice even in that woman in whom he took upon himself the form of a slave” (Holy Virginity 4:4 [A.D. 401]).
“It was not the visible sun, but its invisible Creator who consecrated this day for us, when the Virgin Mother, fertile of womb and integral in her virginity, brought him forth, made visible for us, by whom, when he was invisible, she too was created. A Virgin conceiving, a Virgin bearing, a Virgin pregnant, a Virgin bringing forth, a Virgin perpetual. Why do you wonder at this, O man?” (Sermons 186:1 [A.D. 411]).
“Heretics called Antidicomarites are those who contradict the perpetual virginity of Mary and affirm that after Christ was born she was joined as one with her husband” (Heresies 56 [A.D. 428]).
St Thomas Aquinas, Summa Theologica, III.28.3 (13th century):
"Without any hesitation we must abhor the error of Helvidius, who dared to assert that Christ's Mother, after His Birth, was carnally known by Joseph, and bore other children.
For, in the first place, this is derogatory to Christ's perfection: for as He is in His Godhead the Only-Begotten of the Father, being thus His Son in every respect perfect, so it was becoming that He should be the Only-begotten son of His Mother, as being her perfect offspring.
“Secondly, this error is an insult to the Holy Ghost, whose "shrine" was the virginal womb, wherein He had formed the flesh of Christ: wherefore it was unbecoming that it should be desecrated by intercourse with man.
“Thirdly, this is derogatory to the dignity and holiness of God's Mother: for thus she would seem to be most ungrateful, were she not content with such a Son; and were she, of her own accord, by carnal intercourse to forfeit that virginity which had been miraculously preserved in her.
“Fourthly, it would be tantamount to an imputation of extreme presumption in Joseph, to assume that he attempted to violate her whom by the angel's revelation he knew to have conceived by the Holy Ghost.
“We must therefore simply assert that the Mother of God, as she was a virgin in conceiving Him and a virgin in giving Him birth, did she remain a virgin ever afterwards."
The blessed Mary mother of God, is a perpetual virgin to the glory of God!
