After James and I opened up, I had a long distance intimate friendship with a man we’d met at a party shortly before we moved. James knew about it and although he didn’t like the particular guy (turns out for good reason), it was all above board on our side. Things ended when I discovered that although he was also polyamorous, he’d never told his wife about talking to me on a daily basis.
What he actually enjoyed was doing something behind his wife’s back, and having an intimate relationship that she didn’t know about. He was a narcissist (I finally figured out) and I’m sure that it was his personal and relationship insecurities that drove him to behave in this way.
Ethical non-monogamy has really terrific parameters and ethos in the abstract, but not everyone truly plays by those rules. It takes vulnerability, honesty, and real self excavation. Not everyone is ready to go there.