There's a polyamory movement slowly growing in the US, but ti's still pretty small and not at all organized, so it's basically uinknown/unseen. You have shows like Big Love and Sister Wives championing the mormon shit, but most of it is in the gay community, from what I've seen. Particularly the lesbian community. You'll have a small group of women, usually in college, having a polyamorous relationship with each other until either two pair off and start seeing each other exclusively, or one makes a big case of it and they start to distance themselves from her and she goes elsewhere to find a monogamous partner.
I hang out in an IRC channel that happens to have a lot of GLBT people in it so I tend to hear about this stuff quite a bit. the stuff on transgender (apparently "transsexual is offensive now) and cisgenderism is confusing as shit. GLBT apparently isn't PC anymore either, and a bunch of groups are trying to promote the term "QUILTBAG" instead, because it's supposedly more inclusive. Damn, I know WAY too much about this shit.