I don't believe in this "wiring" idea, but polyamory is certainly not only about a couple branching out or one person falling in love with two people and discovering poly as a result.
There are lots of people who consider themselves polyamorists athough they might not even be in any relationships at all, and there are plenty, like me, who are solo and happy to remain that way. Most solos have multiple relationships but don't need entangled live-in partnerships.
There are a myriad number of ways to live polyamorously. This site has many types of stories but you will only see what is written by the most prolific posters. Tons more people lurk here, and there are even more poly people who never get online at a site like this to share what kinds of lives they lead. All of them have their own unique stories.
Don't start thinking that there is a "usual" in poly.