[whatshisface]

Birth control is what holds the whole thing together, that's the implied part that's not usually mentioned. I grant that IQ 145 people can get together and subconsciously arrange an equitable polycule childcare system, and they don't realize why that would be a problem for anyone else. Moving away from exceptional cases, you have what are more or less roomates, and they manage to stay roomates through birth control.

If you're wondering, "what happens to people that have a good gut feeling for fairness, but who aren't smart enough to subconsciously work out how 5 people are going to take care of a child, and who want kids," then the answer is, they are the vast majority of the population and they're not going to fit the non-monogamous system unless massive inequality (see bronze age) forces them there.