I guess my philosophy is that fairytale monogamy is optimal for the young (say under 200 years or so)

And yet, the vast majority of poly people are well under 200 years old... I doubt they would agree with you on what is optimal for them.

I suppose you could counter that the vast majority of people under 200 years old are monogamous, but that seems more due to monogamy's enormous head-start in modern western culture than due to what is optimal for the young.