Ours developed fairly naturally over the course of about three and a half years.
Friendship, several vacations together and joint home ownership all came before anything romantic. That's not to say there were feelings and desires - babeland.com made a lot of money off of me in 2010 as I fended off "screwing anything up."
All that said - after the third time we hung out as friends, three and a half years before ANYTHING happened, I asked my poly friend out to breakfast to ask how his life worked. Back then, it was just a daydream that I never imagined would come anywhere near reality.
In our day-to-day stuff, we just assume inclusion unless someone specifically asks for their space.