Man, I've gotten a lot of mileage out of this post...
On the 'Her Pertetual Virginity is About Sex' Stuff
Any hypothetical relations between Joseph and Mary would not have defiled Mary because sex within marriage is no defiling act. It is not so much about relations between Joseph and Mary as between a non-Aaronid (non-priest) and a dwelling place of God (her holy womb). The defilement here referred would not be defilement of Mary as a person, but rather of a beit kodesh (holy house- her womb).
And please don't say that 'well, he wasn't there anymore.' Even though the shekhinah glory departed from the Jerusalem temple in AD 30, 66, or 70 (take your pick), Orthodox Jews (and I) will not go on the temple mount for fear to stepping where the holy of holies once was.
However, this does not prove anything. It simply lays down a path for Joseph and Mary to follow- not that they necessarily followed it. In order to take this as proof of her perpetual virginity, you have to first assume her sinlessness. They may not even have thought in those categories.
On Scriptural Affirmation or Repudiation of Perpetual Virginity
There is nothing in Scripture to deny her perpetual virginity, and there is nothing in Scripture to affirm it. It is adiaphora (an area of theological freedom, or at least for sola Scriptura Protestants) in the strictest sense.
The two verses most often used to argue against her perpetual virginity are Mark 6:3 (and its parallels in Matthew and Luke) and Matthew 1:25.
Mark 6:3, as others have pointed out, is not conclusory. The passage refers to brothers and sisters of Jesus, but the word here is adelphos. The term is ambiguous and can refer to almost any sort of relative. In the ancient Greek translation of the Old Testament (the LXX), Lot, Abraham's nephews, is called his adelphos. While, from the passage, it is possible that these four brothers and at least two sisters are Mary's children, it is far from certain. It was simply translated 'brothers' and 'sisters' in the King James (and all subsequent English translations) and, I believe, the German Luther Bible, because English (and German?) doesn't have a single general term meaning 'male relatives' or 'female relatives.'
Matthew 1:25, as one other has pointed out, it also not conclusory. The passage states that Mary and Joseph did not have relations with each other 'up until that time,' i.e., the time of Christ's birth. In English it is perfectly natural to assume that 'until' implies a reversal of circumstances after 'that time.' Not so in Greek. The Greek term heos (until) does not bear this implication; for instance, in the Greek LXX Old Testament, 2 Samuel 6:23 states that Michal, daughter of Saul, had no child until (heos) the day of her death. Are we to assume that the ancient translators of the LXX thought that once she was dead, Michal finally got around to having children? Not at all.
There is simply no New Testament passage that expressly says, or even implies, that Mary did not remain a virgin after the birth up to her death.
On the other hand, the central New Testament passage used by defenders of her perpetual virginity is likewise not conclusory.
When Christ on the cross says to Mary and the beloved disciple, I think it really does imply that Mary did not have other children. Indeed, one of the four brethren mentioned in Mark 6:3, James, was certainly alive and well at this point and went on to become the leader of the Jerusalem church; another brother, Jude, is possibly to be identified with the apostle Jude/Thaddeus/Lebbeus. Why charge the beloved disciple (John?) with responsibility rather than one of these two men?
However, the absence of children does not imply perpetual virginity. All it really does is tip the scales toward the 'cousins' view of Mark 6:3, away from the 'siblings' view.
The other passage used to defend perpetual virginity is Mary's exclamation 'How can this be; I have not known a man,' in Luke 1:34. I wonder, with Catholics, why she would be so surprised if she wasn't expecting to remain a virgin; verse 27 clearly states that she was already engaged to Joseph. If she was expecting to go on to have marital relations with Joseph, we might expect an exclamation more along the lines of, 'Wow, Joseph and I are going to bear the Mashiach?' From this, then, it has been suggested that Mary was a consecrated virgin and her marriage to Joseph a marriage of protection, as was often found in the ancient world.
But, it possible that the angel's appearance implied that the conception would take place prior to the consummation of her marriage, and (as much of the Scriptures are summary) may have been so said in the original appearance. Her exclamation may simply have been a reaction to the idea of a virgin birth in general. I simply can't see how we can honestly exegete a consecrated virginity and marriage of protection from this verse.
Moreover, Raymond Brown, a renowed Catholic scholar and author of Birth of the Messiah, points out that her question is more a literary device used to move along the angel's affirmation of who Christ was from the beginning (over-against adoptionism), rather than a historical record. The genre, he asserts, is polemic in the form of history, not history alone.
On a purely Scriptural basis, then, I see the perpetual virginity of Mary as neither denied nor affirmed. It is adiaphora in the truest sense.
That said, the tradition of her perpetual virginity is extremely early; earlier than any 'one God in three persons' elucidation of the Trinity, for instance (Tertullian, late second century). It first appears in its fullness in the Protoevangelion of James, which came on the scene 120-150. Unlike the other work of second century pulp fiction (The Infancy Gospel of Thomas, not to be confused with the gnostic 'gospel' of Thomas), I think the Protoevangelion deserves our attention.
The Protoevangelion of James actually stands in concord with a great deal of the 'perpetual' interpretations of the text; Mary was a consecrated virgin, Joseph was an elderly man who married her for protection, etc.
Where the text leaves ambiguity, the earliest writing that actually addresses the matter addresses it so as to affirm Mary's perpetual virginity. Even if you reject church tradition (and I know most of you do), the document provides evidence on purely historical grounds. And as for me, as a person who accepts tradition where it does not contradict Scripture (yes, we Protestants who accept tradition do exist), I really can't ignore this.
What's more, by taking the earliest church document to tip the scales of ambiguous texts, we allow Mary and Joseph to follow the theological guidelines laid out at the beginning of the post (prohibition from entering a vessel of God by a non-Aaronid).
And so I stand with Orthodoxy, Catholicism, and all the historic Protestant reformers in affirming the perpetual virginity of Mary (yet with Josiah, as a matter of pious opinion, not dogma).
