GalaGirl - Just wanted to say that your post was so helpful to me! Thank you for all the effort you put into it. I'm going to save this to refer to in future. It really helps to understand that it's (as I assumed) a copout to assume that the person raising the problem is the one responsible for dealing with it, alone.
Since we opened our relationship, and maybe even before, it seemed Ted's attitude was, 'Well, I'm not breaking any rules, so I shouldn't have to deal with or hear about any of your feelings about what I'm doing.' That's a simplification, but that's how it came across. He kept his hurt feelings and adjusting attitudes mostly to himself when I started seeing other people first, and it was then encumbent upon / expected of me that I do the same when he started dating someone else. But I just never did and still don't work that way.
It's much better to acknowledge the hurt, break it down, and decide together, as you noted, who is going to be responsible for what part. It could still come out that the feelings are not reasonable based on the causes, but it's better to figure out together whether they are or not.
Anyway, I need to do a lot more thinking about this, but I really appreciate your answer to my question.