Facebook has, thankfully, moved beyond the gender binary. That is a great step. Facebook should become more accommodating of relationship diversities as well.
Ann E Tweedy of the Hamline University School of Law has this paper available for download on polyamory as a sexual orientation. Here's what the abstract says...
This Article examines, from a theoretical standpoint, the possibility of expanding the definition of “sexual orientation” in employment discrimination statutes to include other disfavored sexual preferences, specifically polyamory. First, it examines the current, very narrow definition of sexual orientation, which is limited to orientations that are based on the sex of those to whom one is attracted, and explores some of the conceptual and functional problems with the current definition. Next the Article looks at the possibility of adding polyamory to current statutory definitions of sexual orientation, examining whether polyamory is a sufficiently embedded identity to be considered a sexual orientation and the degree of discrimination that polyamorists face. After concluding that such an expansion would be reasonable, the Article briefly outlines some issues for further investigation, including potential policy implications and the conflicting evidence as to whether polyamorists want specific legal protections.
Yes, there are people who have been comfortable and fulfilled in monogamous or polyamorous relationships, but that doesn't mean that there aren't people who are polyamorous, just like the existence of bisexuals does not mean that there are no heterosexuals or gays.
Poly people should be as free as anyone else to share love, sex, residence, and, if they want, marriage with any and all consenting adults, without prosecution, bullying, or discrimination. That's why we need full marriage equality.