Read Cecil’s explanation but it seemed plain that the most common-sense interpretation of the Bible was set aside because of the dogma of the Catholic church which in this case has no scriptural support. While Mary was a Virgin when Jesus was born by the seed of the Holy Spirit, there was no need for this state to continue. The doctrine seems to imply there is something sinful about sex. Sex is God’s idea and there is nothing wrong with it when property practiced within the bounds of marriage. Since Jesus was the only one who is divine in the Holy Family, the others could be born the normal way. The New Testament clearly teaches that married people are not to deprive each other, and there is no way Mary and Joseph would have disobeyed God and had a sexless marriage. It takes nothing away from Mary and is totally consistent with the infallible scriptures. 
Welcome to the SDMB, John.
A link to the column is appreciated. Providing one can be as simple as pasting the URL into your post, making sure to leave a blank space on either side of it. Like so: http://www.straightdope.com/classics/a990730a.html
A) The New Testament is not without commendations of virginity, as such, most explicitly in the Book of Revelations.
B) Belief in the perpetual virginity of Mary is not exclusively Roman Catholic.
C) And it can be found in (small o) orthodox writers going back to the 2nd century, long before anything that can sanely be called “Roman Catholicism” ever existed.
Oh, suuuuure, John, confuse the issue with facts!! 
Plus having sex doesn’t necessarily imply preganancy.
Not to mention that we’re told that in her case, pregnancy doesn’t necessarily imply sex.
But since Mary and Joseph predated the New Testament, its rules and recommendations didn’t apply to them.
Oh BTW, here’s the NT’s instructions for marriage, illustrated with Lego bricks:
http://thebricktestament.com/epistles_of_paul/instructions_on_marriage/1co07_01-02.html
Warning: may be NSFW.
Don’t make no nevermind. Judaism is far less supportive of the notion of a vocation to virginity than Christianity is. If Joseph and Mary abstained (and we must here accept the substantial truth of the Gospel narrative, or cadit quaestio), it must have been either because of an unrecorded angelic message, or because Joseph believed that the Mother of the Son of God was something sacred and apard, not to be meddled with, despite the usual Jewish customs and beliefs.
Who else believes this?
I never heard of the the interpretation of the NT that Mary a perpetually virgin until a detailed discussion with an ex-Catholic, but never mind that. Why are those the only choices? Can there not be more mundane reasons, such as impotence on the part of Joseph, or unwillingness on either part?
From Wikipedia (yes, I know, but it’s correct and saves time making multiple cites)
The article also points out that many of the early reformers, up to and including Charles Wesley, also held this view.
So if some folks believe in the perpetual virginity of Mary, where do they think Jesus’ siblings came from?
See, I think Jesus has brothers and sisters who are the respective messiahs of other planets in the universe, and that Jesus is in charge of Earth only. So somewhere out there are people who worship the Daughter of God.
As I recall, they don’t believe he actually had any and that the brothers and sisters mentioned are actually cousins and such.
That is the most usual Catholic-school explanation: some degree of cousins in a very close-knit extended-family situation. An alternate variation on that is that they could be children of some deceased kinfolk, taken into Joseph and Mary’s household and raised as part thereof. Another variant is that they may be children of Joseph by a first marriage.
The issue of how would their marriage exist under the circumstances of true perpetual virginity, would be explained by the churches, like many other head-scratchers in scripture, as working out thanks to a Special Grace granted by God to the persons involved as a special dispensation. Some unorthodox theological thinker could also propose an interpretation that the “aeiparthenos” (ever-virgin) quality refers to a metaphysical condition of Mary, not necessarily related to her transient material state; and finally, of course, one can wonder if as long as we’re buying that God made it so that miraculously a virgin could become pregnant and give birth w/o an effect on either her physical or her spiritual virginity, could we not buy that he could also miraculously make it so that she can have a normal marital life w/o incurring such an effect, either.
In any case, the Catholic and Orthodox Christians past and present are not and have not been absolute scriptural literalists, so the teachings don’t have to square exactly with the text.
From Luke 2:
Doesn’t the word ‘firstborn’ imply she had others after him?
Never quote the scriptures in English as proving a point. That’s a translation of something someone wrote in some other language.
Now, if in the original it said the equivalent of “firstborn,” that would be another thing.
And no, it does not imply a second born. Your only child is still your firstborn, even though there never is a second born.
And, of course, this misses the whole point: since the Church wasn’t “literalist” it didn’t require that it’s doctrines be totally compatible with the scripture as written. They just retconned it. 
The Greek original in that verse does indeed mean firstborn (prototokos=“first-born” in relation to the mother or “first-begotten” in relation to the father). There is another Greek word, monogenes (=“only-born” or “only-begotten”) that occurs in the New Testament. For example, the widow’s son of Luke 7:12 is described as the widow’s only-begotten (monogenes) son. As far as I can tell, Jesus is consistently referred to in the Greek NT as the first-born (not only-born) son of Mary and as the only-begotten (not first-begotten) son of God.
For what it’s worth, prototokos appears to be the term used to translate the Hebrew concept rendered “firstborn” as well as the literal meaning of the word. As may be gained from the Jacob/Esau story, the “firstborn” is the son, normally the first born (hence the usage) entitled to an extra share, the “birthright,” under traditional Jewish inheritance law. Solomon was not David’s eldest son, but he was the son to inherit the throne as holding the “birthright.” The inference that prototokos must mean first of two or more sons is not necessarily true – merely that he was the legitimate son who inherited the birthright. IMO, James, Joses, Simon, and Jude were in fact Jesus’s brothers, the sons of Mary and Joseph – but I’d object to inferring that from prototokos vs. monogenes.
There are multiple explanations, none with any textual Biblical support. One hypothesis (as has already been mentioned) is that Jesus’ putative siblings were cousins or some such and that the Greek word adelphos (adelphes for “sister”) had an intended meaning more like “kindred,” than literal siblings.
Another explanation is that it was figurative and applied to close friends or relatives in the same way that even modern congregations still call each other “brothers and sisters.”
Another proffered theory is that they were Joseph’s kids from a prior marriage.
As I said, none of these arguments have any textual support and are formulated only to defend the doctrine of perpetual virginity which is likewise unsupported by text.
Here is some commentary I once made on this issue on this board a few years ago:
I should have also mentioned back then that both Paul and Josephus refer to James as a brother of Jesus without qualification.
Without commenting on the the nature of Jesus’ own conception, the evidence is pretty strong that Mary had other children besides him.
I think Mary as a perpetual virgin reflects medieval attitudes about female sexuality more than anything intended in the NT. It was definitely not an idea that would have had any value in ancient (or modern) Judaism.