Originally Posted by Flowerchild
Yeah, it's surprising how rare triads/quads/group relationships are...
My guess is that it's not part our biological make-up. We are built to have sex one-on-one after all (at least when it comes to the procreative aspect). Some commentators believe that multiple one-on-one relationships may in fact be the 'normal' state of affairs for homo sapiens and that monogamy is a social construct. But that's another discussion altogether.
So there probably needs to be another level of socialisation skills available to make triads etc. work. These skills aren't hard-wired and therefore require learning. Not everyone wants to make the effort to acquire these skills, or is interested in the first place.