Every couple I've ever known who was in an open relationship involved at least one weirdo and if the OR wasn't the root of their problems, they already had a ton of them to begin with.
A friend was/ is in a "my gf is half-lesbo, so we hook up with girls sometimes together, and I don't mind if she hooks up with girls on the side" deal. They're a certifiable mess.
The other was married/ divorced, and were just as bad. I don't really know them too well.
Small sample size, but enough for me to form an opinion.