This is so, so important! And the rest of this was as well. It's my belief that most couples who are opening up are deeply mired in monogamy thinking and have not taken the time to understand much less work on what it truly means to be polyamorous. This leads to all the toxic behaviors that you mentioned, because they aren't willing to do the work to get themselves into a healthy enough emotional state to be able to handle it.

Sorry that it seems your knowledge base was learned the hard way but thanks for sharing it so clearly. Hopefully it will be of help to others.