I think what *usually* happens is that a monogamous couple (or even a couple with no agreement of monogamy) forms first and then opens up, either by agreement or by one or both meeting someone else they're interested in. And poly *usually* comes from that. It doesn't mean the couple have to be together 15 years and married and feel they need or want more. It could be a couple together 6 months with no expectations of monogamy and then one or both begin dating another/others. It just seems very rare for three or four like minded people to come together from scratch from the beginning because, well, the reason you stated. It's difficult to find that many like minded people at once. From reading posts it seems to me the equivalent of the married couple's "unicorn", which must be frustrating. But it happens I guess, as there is a poster here called sweetlivvie who did just that.
I didn't embrace my poly nature until after marriage so it's difficult for me to understand the perspective of a single poly person. Maybe be open to form one relationship at a time but with the understanding from the beginning that you wish it to remain open? I certainly wish I had done this with conviction when I was younger. Then again, I may not have my wonderful hubby and kids now.
I'd rather be hated for who I am than loved for who I am not.