Elitely, I agree to a lot of logics you have mentioned, but I have my own logics too. I stdied on Anthropology too, out of curiosity.
A female dominated society and multiple sex partner had a reason earlier. The reason is not satisfy the woman's sex hunger, it was keeping the property together. When a man doesnot know its his child or not, he tends to stay together with others assuming its the whole family's property. The best example is what was being followed in Polynesian maternal society.
By having common children, they would give equal importance to all the children, they would not ask to give a piece of land alone, rather remained as collective property for agriculture and hunting to share food.
Secondly, on this guy, I do believe he needs a treatment, both psychologically and physiologically. Get a hardon and sex once in six month is a physiological problem. And only get that thinking of wife being shared must have derived from extreme obsession on his fetish. Yes, he is a cuckold, and he is not alone. But most cuckold you will find are due to their incapability to satisfy, yet a guilty feel that he is not doing the right thing by depriving, thats why get a substitute. In rare cases wives ask for bringing more men. So I feel he needs a psychological evaluation as a couple to check that if she is okay with the concept as he is.