I posted this on a mailing list in response to a question about how folks chose poly and how important was poly to folks' identities. I doubt there's anything new here for anyone who knows me...
I have "always," as far as I know, had the capacity to be attracted in a romantic/sexual way to more than one person at a time. But I went down society's conventional path of believing that I would date around for a while and then settle down with one person. Once I found someone I wanted to settle down with, though, I felt restricted by the idea of giving up the option of acting on my attractions to others. So we negotiated an open relationship. Then things fell apart for a while because I couldn't handle the way my partner got involved with someone else. After some work on that, we got to a point where I had to "choose" between poly (which would at that point involve a lot of emotional work and couples work for us to be able to do poly in a way that worked for both of us) and monogamy (which would be easier but more restrictive and possibly damaging to our relationship in the long run). Somewhere in there, it became clear to me that trying to live poly was important for my spiritual development, so I chose poly.
(I am emphatically NOT saying that poly is important for spiritual development in general. I am very uncomfortable with people who claim that poly folks are "more spiritually evolved" than mono folks, for example. I'm even more uncomfortable with people who make this point of view into some kind of for-profit venture for themselves.)
Being poly is important to my identity insofar as I spend a lot of time in "poly communities" - more time than I spend in queer communities - and I spend some time being something of an educator around poly. On the other hand, my queer identity feels much more deeply part of me. I couldn't change being queer any more than I could transform myself into a seahorse. I can see circumstances where I would be comfortable in a monogamous relationship, though. (Very unlikely ones, since I have several long term sweeties I'm very happy with. But more plausible than transforming myself into a seahorse.)