I think people are generally hostile to the idea of polyamory because it seems like its related to polygamy or some strange form of rationalized cheating on one's spouse. That said, there are people who really aren't satisfied with one person and are very comfortable in relationships with multiple people. One form of this is a "love triangle" with three people that are all in one relationship. The problems are that it can be much more difficult to manage such a relationship, but lets consider some of the frequently cited benefits:
- If being in love with one person is great, couldn't loving two people be even better?
- Having more than one person around (say a stable "marriage" between three people) could be a big help. 3 incomes, 3 pairs of eyes and hands to watch the kids, etc.
Of course, the fact that 2 person relationships are the only legitimate kind of relationship in America truly damages the practicality of alternative forms of relationships. There are lots of other possible combinations, some of which are condemned by society ("promiscuous" people), sometimes with good reason. I'm personally really interested by the idea of a three person relationship, but at the same time I realize that these things happen and are not forced into. Having mutual love between three people is quite a feat.
This site is a great resource for people who are curious about the idea.