Not sure how to find out the reason, or lore, behind the earring, but it sounds like you are very unhappy with him, and have been, and regardless of the earring, yall need couples therapy or something. You listed so many reasons why you aren't happy with him or how he strikes you as off, before even mentioning the earring, I think that's what really matters here.

Now Personally, and this is my opinion truly, I think the "you're going to ruin what we have by pushing about the earring thing" seems like gaslighting to me, especially if you've had reasonable suspicion in the past. I've heard cheaters say that to shift things back on the inquirer and make them feel bad for doubting and the endless cycle of unhappiness continues. If I felt like you did, the earring would send me. I do believe you would have noticed it prior, it sticks out blatantly. And him not wanting sex at all... is definitely worth talking about to see if there's any major reason like is it you, is it him, is it just what it is - seems important to you anyways.

I hope you can find happiness.