I know this thread is a couple yrs old but I've been researching this topic for awhile and stumbled across this, one of the best threads of controversial topics I've ever seen. And everybody generally heeded your request about NO RELIGION.
Let me start this by clarifying that I support polyamory just as I support homosexuality. I am a 27yr old guy who is in love with the woman of my dreams and considering proposing marriage sometime next year. We are monogamous and have considered threesomes and/or "couplesomes" but are a little too shy and frankly, are afraid to try it because we have such a great thing going between the two of us. Any advice on this?
To answer your initial question, America isn't ready yet. Why? Just think about these concepts:
1) New ideas and acknowledgements take time. Think of all the major breakthroughs in science, religious and racial tolerances, and that happened in the last hundred years. Galileo wasn't even officially pardoned for his blasphemies (scientific discoveries) till this century!! It's my predicition that acceptance of polyamory will gradually grow over time. In your lifetime? Very doubtful. But here you are posting threads that get people to think about it and a thought is really all it takes for some people to find acceptance in something new! You are officially part of the polyamory revolution. Congrats!
2) Now to the legal questions you were asking. I will ring the same disclaimer bell you did about not knowing much about law. But I will claim to state some clearcut issues that would at least cause a lot of work for a lot of people if the US were to legalize (or at least legally recognize)it. When referring to benefits, taxes, issues with legal guardianship, etc, you have to remember that our society was BUILT for monogamous relationships and to add an entirely new, complicated family structure for such a small amount of the population would be PHENOMINALLY TAXING. Think of all the paperwork, computer programs, tax options, bank accounts, etc etc etc that would have to be completely redone. The whole infrastructure would have to be gutted and turned inside out in a manner of speaking. Maybe it would be easier to compare this to how our country was built to fuel our automobiles with gasoline and even though we have discovered several fuel alternatives, it is simply too costly at this time to convert our fuel stations to hydrogen fuel cells. Same deal with recognition of polyamory. Too much trouble at this time.
As far as social shunning of polyamory, this is mostly due to religion, which whether you choose to exclude it or not, is also what our country was BUILT on.
If you look at the big picture, major progress is made with our race at alarming speeds and although our lives are not long enough to see much of it, it is somehow reassuring for me to imagine that in 100 years, people will look back on the prejudices we are currently dealing with and laugh.
Gotta get going, but I hope this helped and looking forward to more conversation on this topic.