[kawaii]

I'm operating under the assumption here that a doctrine becomes "false" for these followers of Christ whenever it is not explicitly mentioned in Scripture.

The Eastern Church rejected this doctrine (at east in the way the Roman church states it) for quite a while and it had nothing to do with scriptural support but rather to do with the understanding of the Sin of Adam...

[DogwoodSouth]

Touche', kawaii. I think you point out something extremely important: that Western Christian theology regarding "original sin" (and sin in general) was heavily influenced by the writings of St. Augustine, who was little-read in the Eastern Church until the seventeenth century. Without the existence of "inherited sin" there is no need in Eastern theology for a doctrine which preserves Mary (or anyone else) from inheriting Adam's sin.

It should be noted, however, that Eastern Christians vehemently affirm Mary's sinlessness and there is a widespread (though not dogmatic) belief among the Eastern churches that Mary underwent a "presanctification" or cleansing from a propensity towards sinfulness, at the time of her conception.