Full disclosure - I'm poly and have been for 15 years.
What a judgemental post, OP. I'm sorry you feel that way. I'm also sorry that other people have had bad experiences too. But generalising a whole set of people is really not on.
Yes, there are "grim" aspects of some poly relationships - pushy or selfish partners, boundary issues, abuse, etc. But remember that people frequently have very bad experiences when their partners (or parents) are monogamous and heterosexual.
Most of us are grown ups who behave themselves. But, of course, stories like those don't make it onto TV or MumsNet message boards, do they? And of course now that MN is peppered with Alt-Right plants it's only going to get worse.
For what it's worth, I hated the Louis Theroux documentary. I used to like him, but the poly documentary (and his very judgemental remarks in interviews about it) made me realise that he purposefully picks on grim situations to make good TV. It wasn't representative, even of shit poly relationships. This show is best avoided if you want to learn anything.