There are many drugs with off-label uses that are effective at what they do. Clinical trials are required before companies can market its use. One look around these forums makes this undeniable.
the pde5 inhibitors are not commonly used off-label to treat other sexual dysfunctions. a quick check on pubmed will show that many of the drugs used for, say, psychiatric purposes, but not approved for such uses, as you mentioned above, will provide for an abundance of studies. additionally, a search for anecdotal reports on google will yield the same. however, when one repeats the aforementioned for the pde5 inhibitors and sexual maladies (save for erectile dysfunction) nothing showing efficacy in treating said maladies shows. i did, however, find articles to the contrary, which showed that pde5 inhibitors ameliorated premature ejaculation, which also fall in line with anecdotes found within forums.
one such meta-analysis -
another study -
while i was unable to find a single academic study demonstrating the efficacy of the phosphodiesterase5 inhibitors in the treatment of non-erectile dysfunction related sexual dysfunction in men, i did find articles and case studies that are mentionable. to your credited point, for instance, i was able to find a number of articles that were specific to female anorgasmia being remedied by the pde5 inhibitors, sildenafil, in particular, likely owing to its age. i again, found a few case studies mentioning both men and women, although it was uncertain what was alleviated, as the case studies in point pointed to a number of possible sexual dysfunctions, e.g. loss of libido, anorgasmia, erectile dysfunction, et al., and did not differentiate between.
finally, one that seems to fall in line with your experience, although as an individual case study, again, lending one to believe that the class of drugs mentioned are not widely used for male ejaculatory failure and/or anorgasmia for a reason - inefficacy demonstrated much more often than not.
articles not specific to the serotonergic agents in support of the use of pde5i usage as a remedy for anorgasmia were not found, although that is not to say that they are not in existence.
so, while you may be able to find additional case-study-type literature to support your view, your point was never contested as i have no doubt that tadalafil aided in reversing anorgasmia in your case. the mechanisms behind the reversal have yet to be elucidated as they, apparently, are seen so infrequently so as to not allow for a larger study to be conducted on possible mechanisms. a psychosomatic response is not unfathomable in the cases demonstrated, yours included.