The following originally began as a comment at one of my favorite readers' blogs, The Gordian Knot, but it has, obviously, demanded a place of its own here at FCA.
I noticed you have dialogued a little about the Theotokos, our Lady, the Virgin Mary (that's her full name in Ecumenicalese). I have a few comments.
First, about our Lady's perpetual virginity. I agree with you, and maybe even more vehemently than you, that her virginity was and is vital for the integrity of the Gospel. It was not simply a “sign” of her purity, like a psychological reminder for the sake of human observers, but was in fact a “sacrament” (or demi-sacrament) of her purity as the Theotokos. That is, her unsoiled virginity was an efficacious sign of her purity, simultaneously presenting it and preserving it. Her purity as the mother of Christ in the womb primarily ensured Jesus was never estranged from his Father by the fallen corruption of human nature. Secondarily, though, she retained her virginal purity in order to be the fitting mother of all Christians. While sex in its original form (in Eden) was not impure, the fact of the matter is, sexual intercourse now does continue the line of fallen humanity (regardless whether this is called “original sin” or its less Augustinian Eastern names), not only by commission (intercourse) but also by procession (birth). Baptism regenerates the soul (cf. Rom 6), true enough, but only the eschaton will ultimately transform the carnal (“concupiscent”) body into beatified perfection (cf. 1 Cor 15), a perfection our Lady not only enjoyed on earth but even now enjoys as the great maternal comforter of all the faithful. As Jacques Servais says,
Father Louis Bouyer ... has argued that it is a kind of Monophysitism to accept human motherhood in the order of nature but not in the order of grace: “The most pernicious Docetism of Monophysitism is often the one we do not notice, in particular, Docetism toward ourselves, toward our new life as children of God. . . . The attitude of the Christian who imagines that, at the level of grace, it is sufficient for him to have a heavenly Father, and that he has no need of an earthly Mother, is a very dubious one. Does it not imply that Christian life and ordinary life have to remain on different levels, with nothing in common? There is no vainer illusion! There is no Christian life that is different from ordinary life. Christian life is that life placed under the immediate guidance of God without being cut off from its roots in history.
Beyond all this, insofar as Mary is the image of the Church and, thus, of every Christian, her virginity, like all authentic celibacy, was and is an eschatological and efficacious “sign of contradiction” to the world, proclaiming that once reborn in Christ, a Christian must not be re-united to the world. Her virginity remained intact in order for her to the unsoiled mother of our Lord and, in turn, of his disciples (cf. Jhn 19). The purity of a Christian in whom Christ resides in formation is only an abstraction apart from the incarnated “deposit” of it in the concrete person of Mary.
Second, having said all that, Alexander, I’m curious again to hear the objection EOx have against the Catholic dogma of the Immaculate Conception (apart from its “pious” papist promulgator). Let me quote you:
“If Mary was not sinless and pure, what would that say about the sorts of relationships that we should have with God. ... How can He reside in unclean hearts, much less the unclean womb of an undeserving woman?”
If our Lady really was sinless and perfect in every way, doesn’t that extend to her conception in St. Anne? If not, how or when was she purified enough to bear our Lord? If she was purified at some point after conception, or after birth, or even only at the Annunciation, I fail to see how she could have “pleased the Lord” so deeply that she was announced to be the Theotokos (cf. Lke 1). The dogma of the IC escapes this riddle, because, according to it, Mary was preserved from all sin and therefore never actually (actively) morally displeasing to God. Her “sin” was, for her as an individual, only potential but for her as a human, inevitable. The inevitability of her original sin as child of Adam did not however render her in fact sinful in God’s eyes. God simply delivered her preemptively, by the eternal ordained merits of Christ, from the inevitability of original sin, and her actual sinlessness as a pre-conceived person put no “blockade” between this intervention and her reception of it.
The Annunciation is, therefore, a sort of dramatized, externalized allegory of the Immaculate Conception. Our Lady was favored to be the Theotokos by the grace of Christ and her immaculate nature, even from conception, put up no blockade between this even grander intervention and her “fiat.” To quote from Fr. Bouyer again,
"If there is any Catholic belief that shows how much the Church believes in the sovereignty of grace, in its most gratuitous form, it is the doctrine of the Immaculate Conception ... .To say that Mary is holy, with a super-eminent holiness, in virtue of a divine intervention previous to the first instant of her existence, is to affirm in her case as absolutely as possible that salvation is a grace, and purely a grace, of God."
But again, had our Lady been tainted by any sin at all, I fail to see how she could have received, morally or volitionally, the grace of being the Theotokos as a single, integrated person, not simply as a segment of Mary used by God for the delivery and nurturing of Christ. God did not choose a “phase” of Mary. He chose the whole Mary, from her first moment to her last – that is, from her conception to her assumption – as realized wholly in the mind of God. Her sinlessness at any point in existence would therefore taint her not so much in our eyes but in the eyes of higher creatures – most especially in the eyes of God Himself, an imperfection I believe He refused to accept for the sake of His dear Son. Like every Marian dogma, Mary’s IC was and is fundamentally promulgated and defended for the glory of Christ. Her IC was, therefore, not so much a gift to her, much less a “reward” in anticipation of her sinlessness – what does God “owe” her or anyone else? Rather, the IC was the supreme gift of God *to His Son*, our Lord, for his Incarnation. “A body hast Thou prepared [opened] for me, O Lord” (Heb 10:5). My dad always spiffed up the car before lending it to me for a date. At the risk of being irreverent, I quite sincerely ask, “How much more would God bless His Son with an ‘immaculate’ vehicle into this world?” (Cf. Mth 7:9-11 for this kind of a fortiori theologic.) As I said above, our Lady’s immaculateness was given primarily for the sake of sparing Christ, in his human nature, from the estrangement all children of Adam suffer by normal birth; a secondary and lasting benefit of it is, of course, her fittingness as the Mother of the faithful (cf. Rev 12).
I guess my point is I really can’t understand how the Eastern Orthodox Churches can exalt Mary as sinless and pure (even more ecstatically than many Westerners) without also affirming the dogma of her Immaculate Conception. I hope you know I don’t mean any of this triumphalistically or snidely. I genuinely want to understand the Orthodox objection to our Lady’s totally immaculate status in the merits of Christ. You know where to find me. ;)
 This rejection of “Marian Monophysitism" relates to an essay I have under construction, an essay in which I develop an idea I call “charismatic realism.” I originally intended to send it to Kevin Johnson as an explanation of why Mary still deserves veneration today, even beyond the historical sense of honor more “catholic” Protestants are willing to give her, but I think his email never got it. I sent a copy of it to the Pontificator too and he thought it was pretty nifty. I hope to post sometime before the Second Coming.