I know what you mean.
What my Life Partner and I ended up doing was letting it evolve naturally, if there was some one I was interested in, I would let the relatinship evolve. My life partner is very supporting and participates when it is appropriate.
Not all relationships like to have the third one hanging around all the time.
Of course you always come across those that want to be "exclusive" those are difficult and you need to be strong in your decision.
Most of all if it is the right lifestyle for the two of you then go for it, do make sure that you do talk and never hide anything pre date and post date.
Secrets destroy relationships in a heart beat.
Just a couple of my thoughts.
I own and operate a long standing Alternative Lifestyles Club here in Calgary and we have always been open to supporting such groups like Poly Lifestyles.
Would there be any interest in a monthly get together?
Let me know your thoughts