Warning: Academic Feminist theory incoming. Proceed with caution.
I don't know that I agree that the two examples OP uses are the only public perceptions, or even the most common ones. But I think it's definitely true that the public perception of poly bears little relationship to my own experiences, limited though they be. What I find really interesting about both examples, though, is that they portray very patriarchal relationship structures.
In both cases, there is one man and multiple women, and in both cases the man is seen as the center of power, whether socio-religious or economic. It is certainly worthing noticing, I think, that the two examples that at least one person (and therefore possibly a group of people) clearly feels are representative of public perceptions of poly are very phallo-centric.
I have also seen and heard a lot of speculation about poly culture that includes the idea of swinging and free love, both of which at least in theory seem to equate women with men where sexual agency is concerned. And equality is wonderful, so I have fewer reservations about those depictions. My husband and I don't consider ourselves swingers, but we have been called hippies on more than one occasion, and I guess there's a reason for that.
What I have yet to see in mainstream culture is a scenario where a woman may desire to be the power center in a system containing more than one male partner, without that woman being depicted as a "nymphomaniac" and therefore not truly in control of her sexuality at all. Kind of a "fem-pimp," if that makes sense. It's not that I'm advocating this kind of portrayal, just that it's interesting that it seems not to exist.
Anyway, I'll get down off my soapbox and also add that I would LOVE to see a film made about poly by a person who actually identifies as poly, as opposed to people from outside making stuff that really just perpetuates stereotypes...I'm looking at you, MTV.