As usual my reply to someone's post got a little long, but, as it relates to my thoughts on our current and future relationship configurations, I'd like to re-quote it here.
From my reply to someone's My Intro
My own fear, early on, with Dude was that being in a relationship with me would impede his ability to find a "real girl" (i.e. a primary of his own) - the difference is that he wasn't/isn't actually "looking" for a primary (although not necessarily ruling it out) - this was my own fear. After a while, as our relationship evolved, he said that continuing a relationship with me (in some fashion) would be a necessary part of any future relationship configuration he finds himself in.
Now, lots of things could happen in the future (which is always a true statement). Dude could find a "real girl" who wants to be primary and our relationship could shift into a more secondary model. His new girl might have no interest in a primary type relationship with him (perhaps she already has a primary or doesn't want one) and choose to take a secondary role herself. New girl might be interested in a non-hierarchical model or a "working toward co-primary" type situation (as is evolving between the three of us). In addition, but not expected , the new girl might be interested in pursuing a relationship with either me or MrS as well...
The fact is that this hypothetical new girl will have her own preferences and boundaries, and OUR relationship could be in very different place than it is now (as people and relationships change over time). So, thinking NOW about what things could look like in the future is an interesting theoretical exercise (and I think it is good to be aware of the possibilities) but, until hypothetical new girl is actually on the scene, then no conclusions can be drawn.