This actually gets to the core of another argument that will cause a hellthread. Whether or not Mary sinned.

There's two thoughts there, either all but Christ has sinned and therefore she did- there's also the fact that Mary called Jesus her savior, if she was without sin she wouldn't need a savior- or because she was righteous enough to be chosen for the task of baring the Christ child that she wasn't just exceptionally pure but totally pure.