Saint Augustine wrote that Original Sin was carried through the male sperm. The notion that Original Sin was passed on through the father's seed, somewhat like a spiritual HIV virus, turns out to have been inherently flawed. We must realize, that at that point in history, it was believed that the father, and the father only, contributed what we would today call the genetic make up of the child. What they called the male seed was regarded as containing an entire nascent human being. As a consequence, they regarded any wastage of the seed as tatamount to murder. This explains why masturbation, coitus interuptus and even wet dreams were considered to be serious sins. The role of the woman was solely that of providing the warm nurturing environment for the developing child. She had no genetic contribution to make. Since she contributed nothing to the make up of the child, she could, of course, not be the agency through which Original Sin was passed on. Of course the mother herself was cursed with Original Sin but this flaw in her was not felt to have any bearing on the state of the child.
Now when we link these notions to the Nativity story we get further complications. Mary was believed to have become pregnant through the agency of God. God of course contributed the seed (genetic material) and Mary's role for the next nine months was as a nurturing womb. Jesus was born sinless because of course God was sinless. The stain of the Original Sin did not afflict him. It did not matter that Mary was afflicted with the sin.
This entire theory fell apart about 300 years ago when it was discovered by microscopic studies that the mother did indeed contribute genetically to the child. She of course supplied the egg cell to be fertilized by the male sperm.
This realization seems to have provided a good deal of the impetus for the Roman Catholic doctrine of the Immaculate Conception. If Mary through her egg contributed to the genetic make up of Jesus then she too could pass on Original Sin. The Immaculate Conception solved this problem quite neatly by stating that Mary herself must have been concieved immaculately (without sin) through the agency of the grace of Jesus somehow applied retroactively.