As someone who coaches, mentors and advises many people who are exploring, experimenting or experiencing polyamory, I often encounter individuals who run into the nightmare ultimatum scenario. The scenario is that they are enjoying polyamorous relationships with several individuals, and then one of the individuals decides that they want to be monogamous with them, and asks them to discard all other partners (or, if they are the only two at the time, promise to not seek any others). Some people go for it, and others let the person go, but regardless, this is a very difficult situation, and so the question is how to avoid or prevent it.
Like most things in life, there isn't a catch-all answer or solution, but there are some things that can give one a clue about the future of a relationship. People who might day present such an ultimatum usually don't do it with bad intentions, and most of them initiate the polyamorous relationship believing in all honesty that it's what they want, and only later realize that they are unable to beat their own monogamous programming, or their feelings of insecurity or jealousy. However, one thing that can serve as a clue is the way your partner deals with new partners. When a person enters the life of someone that already has partners, they are typically aware of what they are getting-into, and are capable of handling it. They typically respect partners that preceded them, much like a new employee at a company respects employees with more seniority. However, it is when newer partners join the mix that triggers the hidden insecurities. One of the tropes of monogamy is that if your partner connects with someone else, that is the most dangerous thing to your relationship, as the trope dictates that your partner will likely fall in love with the new person, and discard you for them. While this is inherently not true for a polyamorous relationship, people who are secretly or unknowingly monogamous still have that program running inside their heads. If their partners has a new partner (or sometimes even just has a fleeting interest or intention to have another partner), jealousy rears it's ugly head.
I'm not suggesting anyone does that as any sort of test, but if you are suspecting your partner might run this risk, then one way to clearly see this is observing their reactions when a new romantic interest comes into the picture. A healthy polyamorous partner would feel joy, empathy and compresion, and typically be interest in learning about your new interest, getting to know them, and would typically encourage you to explore the relationship. A partner that's struggling might feel threatened and exhibit defensive or aggressive patterns. Those may not exhibit is simple and plain jealousy, but more subtly. For example, they might suddenly exhibit more interest in you than before, ask or demand more attention and more time together. They might exhibit suspicion or animosity towards the new person, such as questioning their intentions or compatibility. The latter may be very subtle, and if it's there, it might be genuine feelings of being protective of one's partner, and so if you observe this in your partner, be careful not to jump to conclusions. Ultimately, you know your partner best, and can see the big picture. If they do indeed harbor undercurrents of monogamy, that's tough. Many of us are programmed so deeply, that we can't let go, no matter how much we want.
Is there a solution to this? Unfortunately, not really. De-programming and re-education is tough, and it gets harder as we age. A common concept in psychology says that people in their 50s or above cannot be helped with psychotherapy. This is not 100% accurate, of course, and there are exceptions, but in general, this is true. The expression "you can't teach an old dog new tricks" is a colloquial version of the same piece of knowledge. Age 50 is not a hard boundary, of course, but it's a general rule-of-thumb that suggests that around that age, it becomes harder and harder to change patterns of feelings and thoughts, and the same goes for poly/mono.
As much as I hate being the bearer of bad news, this is a reality, and so when I am approached by people or couples in their 40s or above, who are wanting to explore polyamory, I often have to be straight with them, and tell them that their odds are not good. Hopefully, as our society progresses towards more liberal and progressive values, the need to deprogram or reprogram ourselves will become less and less of a need.
