rtyuii: The answer is yes. Something did change in the year before the start of the collapse. But it was, apparently, a positive thing. That year saw the final successful completion of a major academic project that my wife had been working on, off and on, for nearly 10 years, and in which I had been intimately involved during the last 2 years or so. This took a huge load off our lives.

This project had generated a great deal of general stress, and a good deal of inter-personal friction between us. At no time, however, even in the most highly stressed moments was our sex life affected. In fact it was the relief valve for our relationship.
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Herein may lie the answer to your problems:
You say that sex served as a "release valve" for the general stress in your relationship...
Is it possible that you conditioned (not purposefully) your body to operate sexually only in response to stress???
Cognitive behavioral therapy may be beneficial for you, but I would certainly recommend seeing a psychiatrist or psychologist before giving up completely.
It seems to me that if the unconditioned stimulus was stress and the unconditioned response was sexual excitement, then the conditioned stimulus was an erection with the conditioned response being intercourse.
Thereby, a removal of the unconditioned stimulus can lead to decreased sexual excitement and decreased sexual functioning.
It seems to me that you have just given us the biggest clue of all.
Hopefully, this can shed some light in your problems...