Mentions of polyandry are interesting. But we're not talking about a sex-reversed version of the Henricksons, though are we? I mean real-life instances of polyandry have tended to be one man marrying another man's wife, not a woman married to three men who are only married to her, right?

Blame biology.
1 man + 3 wives = 3 babies
1 woman + 3 husbands = 1 baby

huh just tht: cldn't this come into vogue for exactly this biology, as an advantage - population control?
ah:
I can remember in one of my anthropology classes that the only polyandrous society was in Tibet. A woman would marry a man and then she would be considered to be married to all his brothers -- currently living and not yet born. The rationale was that arable farmland was in extreme short supply in that mountainous environment. Instead of going to the system of promeginiture, where the eldest son inherits all, like in European societies, the Tibetan group gave the right of inheritance to all the brothers. Since there's only one one wife, all the next-generation boys are brothers who will eventually all marry one woman, and the land is never divided into plots that are too small to be sufficient. It's land control and population control. I don't think I remember them saying what happened to the extra girls in this arrangement.