Well, to say the least I actually know a couple who is polyamorus. I can tell you, while most people don't really care about it (in my neck of the woods, but we are a weird state anyway). Their son *one of my besties who I went to see the hockey game with since Ig and Val were poops and had to leave* Was not happy with it whatsoever. there are ground rules in Polyamory relationships. I've gotten to know both the girlfriend AND the Boyfriend. And the main couple are married. But out of being open and fair my friends mom *the wife* let me ask questions about their relationship and the "rules" so to say.
They respect the fact that they are married, and that they do love eachother. With that, sex with the opposite is not appropriate. They are allowed to kiss and cuddle, but sex is not important. The romantic aspect, the "love" is the most important part of it. It's the inviting of another person into your close net of family on a romantic level. That's really the only thing. It's when sex and all this other stuff gets added is when things get more complicated. To be fair to the populace, there are a ton of people that think of Poly people as sex hounds, and whores, womanizers etc. This is not true when dealing with ALL relationships of Polyamory. It does in fact break up the traditional home, and causes quite a ton of confusion for kids. I really WOULDN'T recommend a family, cus a child living in that environment might not grow well on relationships (especially if the Poly relation in question is REALLY not 100% stable, they can develop really bad relationship views on how they see couples or members of the opposite sex). To play devil's advocate however, a Poly relationship that has nothing to do with sex and all that other stuff, happens to be like having that really close best friend that you love and care about and would move the world for. But in that sense the word Polyamory could be used very loosely.
Simple fact, it's hard to love one person and another the same. That's just how I see it, it also makes me feel a little strange because competing for the attention and the other 2 might start to feel certain way about it. Personally I say no unless you keep the Romance completely out of it and keep it as a close friend basis, drawing the line very, VERY thick. Does my friend's parents do it healthy? For the most part yes, but the problems are STILL there and they've been doing this for 3 years, and it didn't begin to start working UNTIL those lines were being drawn as thick as they were. The fact of the matter is I think in the end you do start to prioritize one relationship over another, especially when romance is involved. And that's exactly what it had been for my friend's parents they just had to make the boundaries even MORE clearer than they did before. :|