I knew in my teens that poly was very appealing to me - that was over 40 years ago. I was inspired by the novels of Robert Heinlein. I also knew that it would be very difficult to find someone who shared similar views, so by default I lived a mono/married life for many years.
When I divorced, the internet existed, and far more information. I met someone who shared my views, and not only did we become a couple very quickly, we had also met other people while dating who were also comfortable with a poly relationship model. So, we had a primary-secondary style N configuration that lasted a couple of years until our secondaries met others who lured them into primary, monogamous relationships. We were mono for several years thereafter before finding other people - but that's going into another topic.
I think some people know they are either wired for poly or philosophically poly. That may not necessarily translate into real life, however, unless the usual issues and negative emotions (such as jealousy) can be dealt with successfully.