It sounds like they think that this should be effortless if it's "meant to be".
I hope that you can help them understand that a little effort could make a huge difference for the better. I have met people who are monogamous and approach things with this mindset, so this is not a "poly issue" you have here, it's a people issue. It just has aspects that are a little unfamiliar.
Can't you call a time-out and get everyone to just be real for a minute? You don't sound like a bunch of irrational people. You're not throwing tantrums and pouting like children, like someone described in one of the other threads... are you?