Furthermore, how could the Mother of God herself be said to only have one Child when the masculine form of the above word -- ὁ ἀδελφός -- is used to refer to "the Lord's brothers"? Why would Catholics and Orthodox insist so much on the Mother of God's being a virgin, even after giving birth to God in the flesh?
I'll take the issue from a couple of angles. First, I'll explain why the objections to the Mother of God's being an only child (as was her Son) don't hold water. Secondly, I'll show how to reconcile who the women at the Cross were so that the familial relationship between Mary the Mother of Jesus and Mary of Clopas becomes plain. Finally, I'll stress the reasons for (as well as the importance of) the Mother of God's being ever-virgin. I owe a pretty big H/T to Fr. John Hainsworth; his little booklet Mary: Ever Virgin? from Conciliar Press compiles some of this very well. Off we go.
The argument against seeing the Mother of God as ever virgin is that the words ὁ ἀδελφός and η ἀδελφή should be taken at face value; extra-biblical traditions should not be taken as a "trump" over and above what a plain reading of Scripture will tell us, namely, that Mary had other male children who were referred to in Scripture as "the Lord's brothers." Too much, however, is made of this, for the word ὁ ἀδελφός can be used to refer to male kinsmen that are not direct, blood brothers. Indeed, the Septuagint (that is, the Greek translation of the Old Testament which the Orthodox Church uses and which provides for the vast majority of the Old Testament citations in the New Testament over and above the Hebrew text) uses ὁ ἀδελφός to refer to a brother, a kinsman, a cousin, a fellow believer, or a fellow countryman (cf. Gen. 14:14-16; 29:12; Num. 20:14; Deut. 1:16, along with many others). This is useful to know when told that the word ὁ ἀδελφός means "from the same womb," for we can see from the above verses that to say that ὁ ἀδελφός must refer to a male who shares both parents with another person is to deny the biblical use of ὁ ἀδελφός in referring to various other relationships. The "same womb" referred to may very well be that of the tribal matriarch and not that of a woman who, herself, has borne two children. The point is this: While this term does not conclusively prove that the men who were called "the Lord's brothers were not Mary's children, neither can it be pointed to conclusively that they, in fact, were such.
The same applies to the Theotokos. Putting aside the people at the Cross who are named by some gospel writers and not others, let us focus on the woman St. Matthew calls "Mary the mother of James and Joses" (27.55), St. Mark calls "Mary the mother of James the Less and of Joses" (15.40-1) and St. John calls Christ's "mother's sister, Mary, wife of Clopas" (19.25). If we insist that Clopas' wife is the direct sister of the Mother of God and "the Lord's brothers" are direct progeny of the Theotokos, we run into real problems. "James and Joses," the sons of the Mary at the Cross, are mentioned in Matthew 13.55 as οι αδλελφοι σου -- His (Christ's) brothers -- yet they are obviously the sons of a woman other than the Mother of God, and therefore not his blood brothers, but more distant male relatives. Αδελπφος, then, is not "brother" as we think of it, at least not here.
What then, of η αδελφη -- "the sister" of the Mother of God? The Jewish Christian historian Hegisippus sheds some light on this, for he relates (as quoted in Eusebius' Ecclesiastical History iv:22) that Clopas is ο αδελφος of Joseph, the foster father of Christ. Whether the term refers to Joseph's blood brother from the same biological parents or a closer relative, it's clear from what is generally considered a fully-reliable historical reference that Mary, wife of Clopas was not another child of Joachim and Anna, but rather an in-law of some sort to the Theotokos. Thus, through the Matthew/Mark emphasis on the Lord's family ("mother of James and Joses") and the Johannine emphasis on the Theotokos ("Behold, thy mother," and mentioning her at the Cross), we can see that it is entirely possible to claim that the Theotokos was an only child, as we can also claim with her Son.
The question will come, however, as it should: Why should we claim, much less insist, that the Theotokos and her Son were both only children? It should be stated at the outset that Mary's being an only child has nothing, really, to do with the gospel, per se, as the gospel is not about Mary but about her Son. Nevertheless, we would say that, as with other examples within the Bible where a child was granted in an otherwise impossible situation--Isaac to Abraham, Samuel to Hannah, John the Baptist to Zechariah and Elizabeth, Christ to the Theotokos--such a miraculous gift of God is meant to stress that the wonders of God can only be realized on this earth when men believe the word of the Lord to them and He, through His life-giving Spirit, brings them to pass. That the Mother of God would be such a wondrous birth, a sign that the one who would come from St. Anna's barren womb would be the New Tabernacle, the Ark of the Covenant, the Burning Bush, and the Gate Facing East--such an arrival would very much merit a remembrance of God's mercy and power in bringing His Word to pass.
As I said, however, the conception and birth of the Theotokos is important to Orthodox, ultimately, for a more minor reason than that of her first (and only-)born Son. While it is an unwritten tradition of an apocryphal nature that is at stake concerning the Nativity of the Theotokos (which we Orthodox yet hold to be true and a great Feast of the Church), we would say that it is of vital importance to insist that the Theotokos was ever-virgin, for teaching otherwise would amount to denying prophecies in Scripture itself which point to that very thing.
The Prophet Ezekiel wrote of a vision he received in what we now know as the 44th chapter of the book bearing his name. This passage is read during the Vespers service the night before major feasts of the Mother of God:
1And he brought me back to the way of the gate of the outward sanctuary, which looked towards the east: and it was shut.This passage is interpreted by many fathers as a reference to the conception of Christ apart from any man's seed ("no man shall pass through it") because of the holiness of the Prince who has been placed there to eat bread and pass out again of the house of the Lord which is full of His glory. Saint Ambrose asked:
2And the Lord said to me: This gate shall be shut, it shall not be opened, and no man shall pass through it: because the Lord the God of Israel hath entered in by it, and it shall be shut
3For the prince. The prince himself shall sit in it, to eat bread before the Lord: he shall enter in by the way of the porch of the gate, and shall go out by the same way.
4And he brought me by the way of the north gate, in the sight of the house: and I saw, and behold the glory of the Lord filled the house of the Lord: and I fell on my face.
"Who is this gate (Ezekiel 44:1-4), if not Mary? Is it not closed because she is a virgin? Mary is the gate through which Christ entered this world, when He was brought forth in the virginal birth and the manner of His birth did not break the seals of virginity."
The blessed Augustine also stated,
While there are other Church Fathers who speak of these prophecies and others ("At Thy right hand stood the queen," for example), the question comes up among those who, having not lived and worshipped in a tradition who interprets these prophecies in this way, question why we deem it so important to emphasize something that is not, in and of itself, a part of the gospel. Fr. Thomas Hopko is fond of stating that, while the gospel has nothing to do with Mary, Mary has everything to do with the gospel. Fr. John Behr says in his work The Mystery of Christ: Life in Death:
"It is written (Ezekiel 44, 2): ‘This gate shall be shut, it shall not be opened, and no man shall pass through it. Because the Lord the God of Israel hath entered in by it...’ What means this closed gate in the house of the Lord, except that Mary is to be ever inviolate? What does it mean that ‘no man shall pass through it,’ save that Joseph shall not know her? And what is this - ‘The Lord alone enters in and goeth out by it,’ except that the Holy Ghost shall impregnate her, and that the Lord of Angels shall be born of her? And what means this - ‘It shall be shut for evermore,’ but that Mary is a Virgin before His birth, a Virgin in His birth, and a Virgin after His birth."
"...as Origen points out, Christ does not say, 'Woman behold another son for you in my place,' but 'behold your son,' or, as Origen paraphrases it, 'this is Jesus whom you bore.' Those who stand by the cross, and are not ashamed of it, receive as their mother the one who embodies this fertile, generative, faithfulness, and they themselves become sons of God, for they have Christ, the Son of God, living in them (p. 128).The Church has seen in the Theotokos an icon of all Christians individually and of the Church generally. Christ's famous reply to the woman's cry of "Blessed is the womb that bore you" is telling, for it informs our view of the Mother of God: "What is more (μενουνγε, usually translated misleadingly as "rather," cf. Phil. 3.8), blessed is he who hears the word of God and keeps it." Rather than a denial of His mother, the statement redirects us to a full understanding of who she is: the one who most completely heard the word of God and, with her "Let it be," literally kept it. And as she kept it, so are we to keep it, virginally, with no giving of ourselves to another who is not God in order to attempt to bring forth life. To sum up with another quote from Fr. John:
"[In] the preaching of Jesus Christ--the proclamation of the one who died on the cross--interpreted and understood in the matrix, the womb, of scripture, the Word receives flesh from the Virgin. The Virgin in this case, Hippolytus later affirms following Revelation 12, is the Church, who will never cease 'bearing from her heart the Word...' while the male child she bears is Christ, God and man, announced by the prophets, 'whom the Church continually bears as she teaches all nations.' The Virgin Church continually gives birth to Christ by her pure teaching, the gospel proclaimed according to scripture, so that the Word is made flesh in her children. Or, as St. Maximus puts it, 'Christ eternally wills to be born mystically, becoming incarnate through those who are saved and making the soul which begets him to be a virgin mother.'"