There are a few problems with describing polyamory as a sexual orientation. The first of which is that polyamory is not sexual. Polyamory is about relationships, honesty, and intimacy. Look back at the definitions given by Loving More. Not a single one mentions sex. Calling polyamory a sexual orientation is a joke.
Secondly, polyamory is not an orientation. Polyamory is not a physical desire or a feeling. While there is not complete agreement on what polyamory is, there is clear agreement about it isn’t. And it isn’t just an attraction to multiple people. As Shaun pointed out, if you define polyamory as a feeling or an inclination, then half of the country is polyamorous, which is an absurd result. Almost everyone feels attraction for multiple people at the same time. This does not make them polyamorous.
A third problem with describing poly as a sexual orientation is that being poly is nothing like being GLB. Being GLB is about the type of person to whom you are sexually attracted. Being polyamorous is about the amount of people you love. Describing polyamory as a sexual orientation suggests a false equivalence between the groups, and seems like an attempt to coopt the sympathy that the GLBT community has built up.
I'm hearing from lots of poly folks who disagree. I'm going to let them have their say in next week's Savage Love.