I was recently invited to help a friend of a friend answer an objection about Mary's perpetual virginity... [Objector] says that Matthew 1:25 “And knew her not till she had brought forth her firstborn* son: and he called his name JESUS.” Saying that Mary did not have sexual relations with Joseph until Jesus was born, but then after she did. Yet scripture uses the Greek word “until” to have another meaning. It can have the first meaning but it can also mean “up to that point”. 2 Samuel 6:23 “Therefore Michal the daughter of Saul had no child unto [GK until] the day of her death.” Does this mean that after her death she started having children? No.Bible Tidbits is correct that "not X until Y" does not necessarily mean that X will begin once Y is reached. It is an Hebraism, an idiom, and so we need to look past the individual words to see what function the phrase as a whole serves, if it has any function at all. Before I do, let me preface this by saying that as a non-Catholic I have no vested interest in whether Mary was a perpetual virgin**. But I do have an interest in what the scripture has to say about it. So that's what we're going to look at, hopefully without preconceptions.
Unfortunately Bible Tidbits takes the easy way out with its Michal parallel, strong as it looks, because in doing so, they make the phrase meaningless. By making the "until Y" a dividing line that divides nothing, they have removed the meaning from the idiom. Whatever the sentence means, it certainly means something. So what does it mean?
I think there's an idea in play here that we might call "the natural order of things." Michal did not have children until the day of her death. Since we should expect her to have them in the natural order of things, we are talking about something out of the ordinary (a "not X"). But since we would not expect her to have them afterwards, the fact that she did not is natural ("X"). "Not X" ended the day she died (denoted as Y), not because she ceased to not have kids, but because "she had no children" ceased to be unnatural. "Until Y" then serves as a meaningful distinction - it separates the unnatural order that God imposed on Michal from the resumption of the natural order, even though she had no kids on either side of the line.
That this interpretation can fit similar Hebraisms is evidence in its favor. For example, David "took the ten women his concubines, whom he had left to keep the house, and put them in ward, and fed them, but went not in unto them. So they were shut up unto the day of their death, living in widowhood." (2 Sam 20:3). The unnatural order of things was that they were "living in widowhood" even though their husband was alive. But they were certainly not "living in widowhood" after they died, they were just dead. The unnatural order ended at Y.
And "Uzziah the king was a leper unto the day of his death..." (2Chr 26:21). Royal leprosy was the unnatural order of things. After his death, Uzziah did not continue to be a leper; he was then just another dead king.
The idiom then is simply a way of saying the unnatural order "not X" is over once we reach Y. Michal continues to not have kids, while David's wives cease to live in widowhood. So whether "not X" continues through Y is a function of whether "X" is expected in the natural order of things after Y. Make sense?
If we apply this line of reasoning to Mary and Joseph, it works out something like this. The natural order of things is for married couples to have sex. In this case, we have an unnatural order of things - Joseph "knew her not" (Matt 1:25). Again, we have an "until Y," which if this ends the unnatural order of things, would mark the point at which the natural order of things resumed. In that case, there's no need to explain away the biblical mention of Jesus's brothers and sisters (Mark 6:3). The natural order of things is that Mary and Joseph would have kids.
So can we firmly conclude that Joseph and Mary had sex after Jesus was born, i.e. that Mary was *not* a perpetual virgin? I don't know that we can go that far, even though I think that is probably the case. I also don't really think it matters - everything is in relation to Jesus, so Mary's virginity, whatever its value, was most valuable before Jesus was born. I'll let those with a theological dog in the fight take it from there.
UPDATE: "I don't know that we can go that far" is not verbal weaselry, as if I'm trying to avoid offending Catholics. As a former Catholic I have plenty of practice with that. It's just that since one can come up with plenty of theoretical reasons why Joseph continued to not touch her (e.g. he was gay, he was impotent, he suffered castration in a tragic carpentry accident and the kids were adopted) and since the Bible only implies that they had sex***, I drew the difference there between "probable" and "certain." But whether they did or not is wholly unimportant after Jesus was born. Matthew chooses that point for his Y because that represents the end of the matter from a theological perspective. The Virgin had delivered the Savior into the world.
* "Firstborn" does not, as some are wont to claim, assert the existence of a secondborn. Firstborn is a unique concept, wherein certain birth rights are established by the son that "opens the womb" (Ex 13:2) regardless of whether others are born later. That the nation of Israel was God's "firstborn" (Ex. 4:22) doesn't mean that God had other, younger nations. It simply means that Israel was God's special son.
** As Pascal says, "The Bible only speaks of the Virgin's virginity up to the birth of Jesus." Why? Because "Everything [is] in relation to Jesus."
*** When scripture says that sort of thing, it does not do so merely to quench our curiosity, but for one of two reasons: there is either a moral point involved or there is offspring that will be central to the story later. Since neither is the case once Jesus is born, we should expect the Bible to say no more than it does.