But I don't understand the reasoning. Can someone more theologically minded either go through it step by step for me or explain it in a way that is easier for me to understand?
Here is the particular passage (under the section "how voluntary" in the article linked above):
Being a distinct person I am not strictly responsible for the crime of another; the act is not mine. Yet, as a member of the human family, I am supposed to have acted with its head who represented it with regard to the conservation or the loss of grace. I am, therefore, responsible for my privation of grace, taking responsibility in the largest sense of the word. This, however, is enough to make the state of privation of grace in a certain degree voluntary, and, therefore, "without absurdity it may be said to be voluntary"
I am thinking that living in America, a hyper-individualistic country, has sort of blinded me to this "law of solidarity, admitted by common sentiment, which attributes to children a part of the shame resulting from the father's crime."
Although the modern Catechism downplays this, the idea seems to still be present when it says that original sin "is proper to each individual," (CCC 405).
The council of Trent says that it "is in each one as his own (Session V, first decree, point 3)," which I take "proper to each individual" to mean.
I saw this explanation on another forum, which seems to be recapitulating what St. Thomas says to account for it:
"When our father acts as an individual, we are not guilty of his sin; but when our father acts as representative of the family, of which we are a member, then the whole family sins with the father. It's this sense in which we have sinned with Adam, in the sense that he acted not just as an individual man, but as representative of the whole human race."
To summarize (as this is all rather confused):
How is original sin voluntary at all? How is original sin in each one of us as if it were our own? Was it the case that we were all obscurely present in Adam? The father analogy doesn't make any sense to me at all. If my father sins, his sin is his own, even if he acts as representative of the family. Is "acting as representative of the family" somehow fundamentally different?
What really perplexes me about this is that it seems to make acceptable something which I had thought was a modern misinterpretation of original sin, racial shame. It seems like, if we are truly guilty of the sins which we have no consciousness of, as long as they were committed in the past by someone who acted as our representative, then modern white people, or at least the descendants of slave-holders, truly inherit a sin which is somehow voluntary for them (and possibly even present in us as if it were our own?).