Thanks for both your replies.
nycindie, I agree completely. This came up in our last discussion about it last night -- for me, most of the appeal of polyamory is NOT having to predefine roles for people in my life, and letting them just be what they are. My partner is SO SET on having this specific configuration (and all of a sudden, too, this came completely out of the blue) and I agree with you that it is just a completely unrealistic expectation.
Of course when I said as much to him he was really hurt that I was calling his dream "bullshit" (his word, not mine). I am thinking before I give up completely I want to talk to him about a lot of practical issues he seems to be avoiding.