He doesn't even want to find an existing situation, he wants to build it himself out of people he picks!
No, he has never seen this work. I guess we know 1 poly family in a similar situation, but he constantly uses them as an example of people who are totally dysfunctional and he doesn't even want to be friends with them!!! As far as I know he has NOT met someone willing to do this, in fact his 2 most serious long term relationships before me (including a marriage) both ended because the women had decided they wanted to be completely monogamous.
I have never asked him for monogamy, I admit I have asked for a lot of boundaries and really exhausted his patience at times, but I agree with you, holding out for this unrealistic fantasy is just nuts. NUTS.