I can see the difficulties with the disparities in "amount of libido" in you versus A; just one of the reasons why poly might be helpful to you, perhaps.
I understand that you and A are kind of in limbo at the moment as to how well you can agree to this poly thing: DADT and LDR and many other factors. You just do the best you can do, and focus on the here and now.
"I guess that we're still in discussions about relaxing the 'sexual exclusivity' element of our marriage (nothing else would change)."
Ah ... and what of the "emotional/romantic exclusivity" element? (Not trying to poke you here; just curious.)
Interesting; sounds like if the "poly question" were removed from the equation, A would be happy as a clam in the relationship he has with you. That's actually a good sign!
"Does anyone ever really *know* that they have a poly mindset before they suddenly find themselves, as I did, in a situation where you realize that you truly romantically love more than one?"
In my poly forum experience, it really varies from person to person. Some people "decide that they're poly first," and then look for (a) poly partner/s. But it's possibly *slightly*
more common (okay but given the Choosing Polyamory Before Identifying a Second Lover?
thread, my jury's out) for people to say, "Uh-oh, I'm in love with a married person, now what do I do," or something like that, and then (often with the help of Google and whatnot), discover that "polyamory" is the "emotion" that they're "experiencing." This is often because people just don't hear about poly until they're "up against the wall."
As polyamory becomes more widely-known, and a part of mainstream culture, I'm sure more and more people will decide they'd like to live polyamously first, before they set out to implement the desire. Which seems to be a good thing.
Such is my perspective for what it's worth.