On December 9, the Holy Orthodox Church, commemorates the conception of the Theotokos by her parents, Sts. Joachim and Anna. The Roman Catholic Church uses this feast to affirm the "Immaculate Conception" of Mary which, in 1854, was officially promulgated as a dogma of the Church, which means that assent to it is a prerequisite to salvation. Needless to say, the Orthodox Church remains opposed to such a dogma, simply because the Roman Church and her progeny from the Reformation erroneously, following St. Augustine, believe that conception of children and, by extension, sexual intercourse is a transmission of the "stain" or guilt of original sin.
Fr. Thomas Hopko replies to this belief and expounds the Orthodox position thus:
The Orthodox Church affirms original sin. Orthodox theology teaches that all human beings, including the Virgin Mary, who is a "mere human" like the rest of us--unlike her Son Jesus who is a "real human" but not a "mere human" because He is the incarnate Son and Word of God-are born into a fallen, death-bound, demon-riddled world whose "form is passing away" (1 Cor 7:31). We are all born mortal and tending toward sin. But we are not born guilty of any personal sin, certainly not one allegedly committed "in Adam." Nor are we born stained because of the manner in which we are conceived by the sexual union of our parents. If sexual union in marriage is any sense sinful, or the cause in itself of any sinfulness or stain, even in the conditions of the "fallen world," then, as even the rigorous Saint John Chrysostom (see On Titus, homily) has taught, God, is the sinner because He made us this way, male and female, from the very beginning.
--The Winter Pascha, pp. 42-3.