Barring your complete disregard for logic, Hot Mega, and disregarding the slippery slope argument you made (which holds no water in any rational debate)...
Who's to say that a polyamorous relationship is any more messed up and asinine than the multitude of malignant and loveless relationships that monogamous heterosexual couples have?
Really, if consenting adults love each other and want to be in a relationship, why not let them? As long as no one is getting hurt, there's no reason to limit them just because it deviates from the norm.
Besides, using your logic, we should have never let women vote; after all, if we let them womens folk vote, what's to stop kids and animals from voting?