Heh, it's funny, I suppose it's pretty obvious that my suggestion would've been to consider opening up more rather than the reverse. But you guys are not me and I certainly can't tell you what's right for you. I'm glad you found my words of use.
I would say that the sort of closed, three-person polygamous marriage you're seeking is a form of polyfidelity (closed relationships between more than two people), which is a form of polyamory (love between more than two people), so you're still in the family in my book.
You may continue to get people questioning your method, since more than one person here had started with the goal you did and then moved to a more open model (as per RP's story above). Still, I would encourage you to continue viewing our community as a potential source of help even if you chose not to identify with it per se, since 1) we have members in three-person relationships and people who practice polyfidelity, and 2) you say it's helped so much already.
I do have to ask -- why do you consider sexual exclusivity such an important condition for your new partner when you have two examples of marriages -- yours and that of your friends -- that have functioned without it for so long with no diminishing of love or commitment? It puzzles me.