I don't know if I've always been poly - certainly when I was younger I didn't see why loving lots of different people at once would be a problem. As I started dating (and met my partner) monogamy seemed the only way - social conditioning told me that this was the only way to feel secure and loved.
As we settled into our relationship (we've been together 8.5 years now) the security came from within - we began to discuss seeing other people and decided that the strength of our commitment didn't mean we needed to be exclusive to feel 'secure'.
Ideologically we are both feminists and don't feel we own each other or are each other's property - poly fits in with this very well.
So we've sort of come round to being poly, we've moved from one end of the spectrum to the other.