I find sociology to be very interesting. Especially when it comes to literature and movies. A VAST majority of movies about swinging or poly are a cautionary tale. They typically end with someone getting hurt badly or discovering that they were really just sick in the head. Same with BDSM or any other "taboo" lifestyle.
I've observed the same thing.
If we take the example of gays / bi's and lesbians in literature and film, there appears to be a very strong trend line. The earliest examples typically have simply ghastly endings, for example.
Take Gore Vidal's novel,
The City and the Pillar, for example.
" ... written in 1946 and published on January 10, 1948. The story is about a young man who is coming of age and discovers his own homosexuality.[1]
The City and the Pillar is significant because it is recognized as the first post-World War II novel whose openly gay and well-adjusted protagonist is not killed off at the end of the story for defying social norms. ...."
....
" .... The resolution of their relationship comes again in New York, where they end up on the bed in Bob's hotel room. But when Jim finally thinks he has attained what he wants and moves closer, grabbing his "sex", Bob panics, is outraged to be thought of as gay, and even punches Jim in the face. The two struggle and Jim wins because he is stronger. In the original version, Jim is infuriated enough to murder Bob while in the revision he rapes Bob and then leaves the room ...."
http://en.wikipedia.org/wiki/The_City_and_the_Pillar
The early gay / lesbian novels and films I've seen have all portrayed gay / lesbian people as deeply disturbed, murderous, ... well, crazy people.
Over the decades, gradually, we became just folks -- regular people, more or less -- in fim and novels.
Non-monogamy, I think, has been -- and probably will continue -- to follow this very same trend line or pattern.