Val and I were talking this week about polyamory as a sexual orientation. There was an interesting argument on Savage Love awhile ago about this very topic as many poly people seem to want poly to be considered an orientation in the same way straight, gay, or bi/pan/omni are. I think that is silly. Polyamory isn't an orientation as orientation describes what gender(s) a person is attracted to. It is still important though, particularly since it describes nearly all of the human population.
That last sentence might seem a little controversial but I think it is clearly true if you consider poly as a mindset rather than as a set of rules one happens to follow at any given time. People fall in love with multiple people at a time. They experience lust for more than one person at a time. This isn't universal of course, as there are people who don't experience those things at all and there are those that only experience them for one person at a time but they are by far the minority. The great majority of humans are very much capable of lust and love with multiple people but mostly they have rules that try and fail to curtail these natural tendencies. Of course the majority of people aren't actually into triads or quads or line marriages or anything of that sort but they are perfectly capable of experiencing some sort of powerful romantic affection for more than one person.
The problem is just terminology. Polyamory as a practice, a lifestyle, or a relationship type is not widely practiced. Polyamory as a natural inclination is the normal state of humanity and the fact that our culture is so utterly dedicated to squelching it and yet fails to spectacularly at doing so illustrates this clear as day. If poly weren't so normal, ingrained, and unavoidable then infidelity rates wouldn't be in the ballpark of 50%. (And that is the infidelity people admit to.) Poly, like sexual orientation, isn't something people can change just by wanting to but it isn't an orientation and trying to shoehorn it in there is just making a muddle of things.
A big part of why this is an issue is legitimacy. People with nonmainstream relationship styles want to be considered legitimate by the rest of the populace. One of the ways that happens is by leveraging terminology and since most people now accept sexual orientation as legitimate it is natural that people would want to ride on the coattails of the gay rights movement. I think that misses a fundamental point though, and that is that if we try to force each individual thing through as an orientation we give that word and its official meaning way too much power. Better to simply try to get across that every set of desires among consenting adults is fine and avoid the pointless battles over syntax.
The other way people typically try to legitimize their relationship styles or desires is by finding examples of them among other animals. I think this is an extremely suspect line of reasoning since animals regularly engage in male on male battles to the death for sexual ownership of females and we don't accept that as natural and okay. We also should not tie legitimacy to animals because the idea that tying a person's hands to their headboard during sex is wrongbad because we don't find animals who do the same is absurd.
The argument for legitimacy should start and end with "I am an adult, this is what I like, and everybody involved is consenting." No other argument is required.