If you deny the perpetual virginity of Mary, you'll have a hard time finding support in the tradition - even among Protestants.
One such example is Matthew 12.46: "While he was still speaking to the people, behold, his mother and his brothers stood outside, asking to speak to him." Other examples include Matthew 13.55, Mark 6.3 (this one mentions sisters), John 2.12, John 7.3, John 7.5, John 7.10, Acts 1.14, 1 Corinthians 9.5, and Galatians 1.19.
Here's a traditional response:
There is another passage usually brought up as a evidence that Mary was not perpetually a virgin, Matthew 1.24-25: "When Joseph woke from sleep, he did as the angel of the Lord commanded him: he took his wife, but knew her not until she had given birth to a son. And he called his name Jesus."
To say that Joseph "knew her not until she had given birth to a son" (my emphasis), many Protestants argue, seems to imply very strongly that Joseph did "know" Mary after she had given birth to Jesus. Another traditional response:
“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."
Who's this Helvidius that keeps getting mentioned? He was a 4th century theologian who taught that Mary was not a virgin after the birth Jesus and based this belief on the very same verses that we have brought up. Helvidius' view was immediately condemned in the Church as newly-invented blasphemy, with a famous defense of Mary's perpetual virginity given by St Jerome in his work The Perpetual Virginity of Blessed Mary: "Pray tell me, who, before you appeared, was acquainted with this blasphemy? Who thought the theory worth two-pence?" (18)
The explanations quoted above in defense of the perpetual virginity of Mary, while accurate, are fairly poignant. So who are they from? None other than the great Protestant Reformer John Calvin.
The first quote is from from his Commentary on Matthew, Mark, Luke, Vol 2, Matthew 13.53-58 and Mark 6.1-6, and the second is from his Commentary on Matthew, Mark, Luke, Vol 1, Matthew 1.18-25.
For full disclosure, Calvin seems to indicate in a sentence following the second quote that, while he doesn't think there is Scriptural warrant for denying the perpetual virginity of Mary, he doesn't think the question is of much importance either:
But notice that Calvin thought that "Helvidius displayed excessive ignorance in concluding that Mary must have had many sons, because Christ’s brothers are sometimes mentioned," (my emphasis) – the very same conclusion that many Protestants today make from those passages.
And regarding the other common Protestant argument from Matthew 1.24-25, Calvin thought "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" and that "[w]hat took place afterwards, the historian does not inform us. Such is well known to have been the practice of the inspired writers."
Calvin was not alone among the Protestant Reformers in defending the perpetual virginity of our Blessed Mother.
Martin Luther wrote:
"Christ, our Savior, was the real and natural fruit of Mary's virginal womb . . . This was without the cooperation of a man, and she remained a virgin after that. […] Christ . . . was the only Son of Mary, and the Virgin Mary bore no children besides Him . . . I am inclined to agree with those who declare that 'brothers' really mean 'cousins' here, for Holy Writ and the Jews always call cousins brothers." (Sermons on John)
Huldrych Zwingli wrote:
Even John Wesley, in 1749, wrote:
Calvin, Luther, Zwingli, and Wesley were simply maintaining the long-standing traditional belief on the matter. Here are just a few witnesses:
St Gregory of Nyssa, On the Holy Generation of Christ, 5 (4th century):
St Augustine, Sermons 186.1 (early 5th century):
St Thomas Aquinas, Summa Theologica, III.28.3 (13th century):
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."
Of course, most Christians today still believe in the perpetual virginity of Mary (think Catholics and Orthodox). The practice of most Protestants today of denying the perpetual virginity of Mary is a fairly recent innovation – a peculiar historical aberration, particularly since certain Protestants, such as evangelicals, would consider themselves to be conservative Christians – that can't even find historical precedent among the primary magisterial Reformers; for that, one can only look to a hand-full of 4th century teachers who were otherwise universally rejected as heretics.
Catholics, on the other hand, have maintained the 2000 year tradition that our blessed and holy mother Mary was indeed not only a virgin before Jesus' birth but also perpetually a virgin thereafter.