VII. Soul of the problem
We now consider the consequences of the third assumption (Section I) on which the doctrine of original sin is based, namely, that Adam and Eve were punished in a way that would affect all their descendants. The argument against such a hereditary punishment from justice is itself compelling. How can a God who is just punish the entire race for a sin it did not commit?
Rabbi Kushner extends the justice argument to Adam and Eve themselves:
A God who punished people so severely for breaking one arbitrary rule was not a God I wanted to believe in, especially since the story seemed to suggest that Adam and Eve had no knowledge of what good and bad meant before they broke the rule. 
We, however, will consider the issue on a more fundamental level.
Original sin is said to be passed down by "generation," a term originated by Augustine.  Augustine maintained that a connection exists between the physical and spiritual parts of a human being such that damage can be transmitted from one to the other. The "nature of the semen from which we were to be propagated" is "shackled by the bond of death" so that it transmits to every human being at conception a hereditary contamination by Adam's sin.  The only person not so contaminated by sin was Jesus because he was conceived without the intervention of semen. 
In the light of modern scientific knowledge, however, the latter argument falls apart. Science at the time of Augustine had no inkling that the mother also contributes to the conception of a new human being. The existence of the ovum -- Mary's contribution to the conception of Jesus -- was not discovered until 1827,  150 years after van Leeuwenhoek discovered spermatozoa with his microscope.  If the male germ cell (of which Augustine was ignorant) was contaminated by original sin, then the female germ cell (of which Augustine was also ignorant) would also have been contaminated by original sin. After all, Eve ate the fruit of the tree before Adam did.
Augustine himself recognized a problem with his concept of generation, but never did offer a definitive explanation of how sin could be transmitted hereditarily to the incorporeal soul. He did not champion any of the various opinions advanced at the time regarding the origin of the soul,  none of which resolved the issue. Instead, he dismissed the problem as unnecessarily clouding the danger of the Pelagian view. 
The Catechism likewise attempts to dismiss the problem by declaring it "a mystery that we cannot fully understand." 
Contemporary apologetics of generation is noticeably incoherent. Take Schmaus, for example:
This does not mean, of course, that Adam's sin is inherited like some sort of biological element ...; rather, his descendants share in his sin on the basis of their biological connection with him. 
Thus, generation is not biological per se, but proceeds somehow from the biological ancestry.
Let us consider the implications of this theology.
According to Roman Catholic understanding today, each new individual human being comes into existence through procreation, whereby the parents unite to create the new body from pre-existing matter and God directly creates the new soul. The resulting human person is composed of a physical body and a spiritual soul, two ontologically different realities neither of which can be deduced from or reduced to the other.  Therefore, original sin must be passed on either in the creation of the new body or in the creation of the new soul.
Original sin cannot be passed on through the physical action of the parents in the creation of the new human body because it is not a physical condition. As Schmaus correctly asserts, it cannot be a genetic factor.
The putative original sin is a spiritual condition. Ontologically, it can only be passed on in the creation of the spiritual part of the new individual, an action that is not governed by either the bodies or the souls of the parents. This puts God in the position of creating the new soul in a corrupted or tainted condition. Aquinas raised this very argument. 
God's creative action can hardly be limited or defiled by some transgression of one of his creatures. That would give man power over God rather than vice-versa -- truly an absurdity. But such is the logical extension of traditional original-sin theology.
The alternative -- which seems equally invalid -- is that God himself changed his design of the human soul because he was unable to foresee how the prototype would turn out.
In his reply, Aquinas begins with the necessary assertion that original sin is "nowise caused by God." The soul cannot be tainted through being created by God and it cannot be tainted through being infused by God. The only alternative left, which Aquinas curiously adopts, is that the soul is tainted by the semen-corrupted body into which it is infused.  And that contention -- that matter can affect the essence of spirit -- is ontologically impossible. Aquinas himself affirms the impossibility elsewhere: "the incorporeal cannot be affected by the corporeal." 
None of the alternatives is possible; ergo, "generation" is impossible.
"What is original cannot be sin, and sin cannot be original."