Originally Posted by Hades36
I mean, from a practical, survival kind of standpoint, having more than 2 people in a relationship seems really beneficial. But there also seems to be a number, a tipping point as it were, where the relationship would be too large and clunky, at least with regards to intimate connections running smoothly. I imagine at that point the group would splinter off somehow and form a new family.
Trust the aspiring social scientist to start the nit-picking!
If we are talking of purely human evolution, there are many explanations as to why monogamy is the most popular (even in societies where some form of non-monogamy is condoned, it tends to be a minority relationship model) relationship form. Of course, when we are talking about monogamous vs. polygamous societies, we need to acknowledge that most known forms of culturally-sanctioned non-monogamy are in practice polygyny (having many wives) of the rich elite men. Some anthropologists have put forwards an argument that wives in most societies are akin to any other form of goods to be exchanged and accumulated among leading class men. This theory has some historical support from the fact that the ideal of female sexuality being strictly marital in expression has always been more heavily policed in the higher echelons of society. During much of Western history, rape was a crime against another man's rights of ownership, not against the woman who was raped.
Comparing monogamy and polyamory is hard because polyamory is a form of relating, whereas monogamy refers to the institution of marriage specifically. The opposite of polyamory would be monoamory, such as the opposite of polygamy would be monogamy. As to your point of relationships becoming too unwieldly with too many participants, this is only in assuming that each new partner would join the existing family unit in what could more aptly be termed as 'group marriage'. Since I guess the majority of polys are not in closed poly-fi arrangements, the practical limits of how many partners you could possibly have tend to be a bit different in origin.
Additional differences: lifetime monogamy vs. serial monogamy; double vs. single standard; monogamy as an ideal and monogamy as a practice; hierarchal vs. egalitarian polyamory etc.