Based on all the available evidence (with 1-4% of people getting PFS who take finasteride), the most likely etiology of PFS is that the gene for 5a-reductase type 2, SRD5A2, is epigenetically silenced in key tissues responsible for sexual function, including in the brain, through DNA methylation. Gene methylation is a well-described process that is known to occur with many genes and with this particular gene as well. If the right conditions are met, methylation can happen in seconds in millions of cells throughout the body. A single pill of finasteride can definitely trigger such a reaction.
The reality is that men complain of PFS, so vaid or not, it is real to those men who experience it; however, since neither I nor many of my colleagues have seen it, the frequency seems less than the 1-4% that you have referenced. If you think about it, why would I continue to prescribe finasteride as I don’t make any money from doing it and if I didn’t more of my patients would become surgical candidates and I would make more money from doing their surgeries. The answer is obvious, that the drug works for many men with minimal or no side effects and I care about the patient’s welfare. Impotence is common in men without finasteride on-board, about 20% in men in their 20s, 30% of men in their 30s, 40% of men in the 40% and so on. I appreciate the time you took to write this post to me, but although I believe that PFS does occur, but it may not be as prevalent as you state.