Okay, philosophical argument is all about words and their meaning. Polyamory could be considered against "accepted principals of right and wrong behavior" (the definition of morals). Morality falls back on tradition and, often, the religious principals of the majority. Therefore, polyamory technically could be considered immoral in western society.
Morality is crap. I think what you really want to discuss is whether or not polyamory is ethical. Ethics is the philosophy of how people should act. It relies on logical arguments to determine the best course of action. I'd argue that polyamory is an ethical choice for me. My ethical argument would look like this:
1) Something is ethical if it creates the greatest good for the greatest amount of people (i.e. utilitarianism).
2) Polyamory creates the greatest happiness for some people without severally impinging on the happiness of others (first-hand experience).
3) Therefore, polyamory is an ethical choice for some people (i.e. me).
Incidentally, monogamy would make me unhappy in the long run. My unhappiness would most likely make the people around me more unhappy. Therefore, for me, monogamy is an unethical choice.
Just my 2 cents and a minor in philosophy.