As to the thread in question, I agree that the best way to "explain poly" to one's parents/family is through the passage of time (and thus the proof that it's not a harmful relationship). That and answering the family's questions as best as one can. But when the family first hears about it, sure there's a chance some will be shocked and/or offended, no matter how well the news is presented. People just aren't yet prepared to hear about poly in today's social environment. Hopefully the social environment will improve in the future.
There are lots of people who "do poly badly," just as there are lots of people who "do mono badly." We could argue all day about which romance style is best for the kids, or whether either is. It's just not the type of thing one can "prove," even if a lot of studies are done and the evidence is in. And right now, poly is a decidedly under-researched lovestyle.
As for the way a few of the posters were acting on that thread, I'm afraid we have to expect that as a part of the conservative bias that still exists in our society. I don't condone it, but I advocate being aware of it (and being on one's guard, for that matter). Sadly, I have to say that I was surprised that the thread didn't explode into something much uglier. I thought the responses to the conservatively-biased posters were fine and appropriate and couldn't have been a lot better. Sometimes shiz just happens.
Sorry such an unpleasant experience happened on that thread. At least you know that on Polyamory.com, you don't have to worry about being hit with those kinds of condemnations. Take some comfort from that knowledge, anyway.
Love means never having to say, "Put down that meat cleaver!"