OldGuy, I do see what you're saying. If a relationship is healthy, it's healthy. However, I would argue that most monogamous relationships, even healthy ones, are founded on a mutual, generally unspoken understanding that jealousy, insecurity, etc. are in fact there. In healthy relationships those things just don't get triggered, so it seems like they don't exist. But they are in fact still fundamentally there. Maybe this is a discussion for another thread, I've been considering starting one on jealousy and insecurity.
And by the way, the things you listed that indicate a lack of emotional maturity...personally, I would say those things indicate the opposite.
I disagree with this. What I think leads to a lot of this prejudice is that what are generally known as 'poly principles' are things that tend to make healthy relationships. You know: communicate, work though your emotions, try to be more objective and not let your own insecurities get in the way too much, etc.
The problem is, that whilst these are principles that are very strongly associated with poly, they are not Just poly ideals. They are the things that make any healthy relationship/individual. They exist outside polyamory and many mono couples practice them, many mono couples learn about them in couples therapy, etc..
So, I think, the reason that poly people seem to be more healthy and well adjusted is because these ideals are more firmly taught and then applied in 'poly society'. Poly people don't automatically adhere to these ideals or practice them, if they did, the Poly Relationships section of this forum would probably have a lot less poly people asking for advice.
Personally, I think, if people were taught more about these sorts of general relationship principles early on, then there wouldn't be so much poly people vs mono people, but more ignorant people vs people who are happy to learn and generally expand their knowledge.
People ask me about poly a lot, and a lot of the mono people that I have talked to have then taken on some of the general ideals and used them to make their relationship work. Some of them already practised all of those ideals in their mono relationships. Several poly people I've come across, don't use any of those ideals, don't communicate at all and then end up with relationship shambles.
It's not the relationship style, it's the people involved, how they go about relationships, and, often, how they've been taught to go about relationships.