Why is this question posed anew by the doctrine of the Immaculate Conception? The economy of salvation is such that the Jewish culture had to mature, through repeated bondage and liberation, so that the standard of ritualistic purity at the core of Judaism could culminate in the Immaculate Conception that men would pray for, recognize (on some mystical level) when it happens, and protect from defilement, as the Protoevangelium shows.
Very reasonable. So if I am understanding you correctly, the Catholic belief is that the IC awaited a "ripening," so to speak of the Jewish people -- which is similar to the Orthodox view. The main difference is that at that moment, God acted supernaturally to cause a conception (that of Mary) that was different from ordinary human conception, even though it was the result of a union of man and woman.
Something about this still bothers me -- it strikes me as having parallels to the Protestant view of the conception of Christ as a pure act of God's will (just backed up a generation), but I'll have to think about it before I commit to an observation to that effect.