I think the main problem is that the increase in complexity (for the characters themselves, let alone the person who has to manage describing what they're doing) isn't linear. Dealing with three characters in a relationship isn't twice as hard as two, it's more like nine times as hard. With four it'd be sixteen. Gods help you if you ever have to go past that, it probably turns into a logarithmic progression at that point. In Zeta Cygni, polygamy is legal, but any combination of more than four people has to go before a review board before adding anybody else to make sure everybody involved can actually deal with that action.*

--G.
* That second part is probably a joke. I hope it is, anyway.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
zgryphon at that email service Google has
Ceterum censeo Carthaginem esse delendam.