Stuart L. Koehl, a Greek Catholic friend of mine, speaking great sense on the question of the Virgin Mary and Theotokos’ conception and sinless life:
[While foreknowing that the Virgin Mary would say ‘yes’ to bearing Christ, God] must also preserve her absolute freedom to reject the mission, otherwise she cannot fulfill her role as the Second Eve. And the meaning of “immaculate conception” depends largely on your understanding of the term “original sin”: does man bear a stain of the sin of Adam that somehow renders his nature corrupt from birth? Or does man bear the consequence of Adam’s sin, which is mortality? [The latter is the Orthodox view].
Mary died, ergo, she still endured the consequences of Adam’s sin. But Mary was also preserved from sin throughout her existence. Is this due to Mary’s conception being ontologically different from that of other human beings? Or is it due to God placing his protective grace over and through her from the moment of conception? [The latter is the Orthodox view].
It is difficult to remember, and all too easy to forget, that the Western thread of anthropology saw Adam’s sin as being transmitted through procreation (the [Catholic] Church would rather forget that, these days, but the fact is, it taught it for centuries, and it deeply colored the Western Church’s view of sex). So, in Western eyes, if Mary was conceived as other women, then she herself would be tarred with the sin of Adam; therefore, her conception must have been . . . different.
On the other hand, if you take the Eastern Christian position that man suffers from the effects of Adam’s sin, but not the stain, and that this leads man to develop disordered passions that lead to actual sin, then Mary can be conceived as other women, and then protected from all sin from that moment forth, by God’s preeminent grace.
This view tends to be borne out in Eastern liturgical texts. While on the one hand, Mary is called “all-holy”, “all-pure” and “without stain”, at the same time other texts, like the Paschal Hymn say:
Having beheld the resurrection of Christ,
Let us adore the holy Lord Jesus,
Who alone is without sin [also translated “The Only-Sinless One”]. . .
While the funeral service (Panahida) says:
For there is not a man who lives and does not sin,
In thought, or word or deed,
And You [Christ] alone are without sin,
And to You we give glory. . .
So, Christ alone is ontologically without sin, while Mary’s sinlessness is derivative of Christ’s grace and Mary’s perfect cooperation with it.
This explains how Eastern saints such as Maximos the Confessor could believe Mary was preserved from all sin from the moment of her conception, without believing that Mary’s conception itself was different in any way from that of other humans–he simply had a different view of original sin.