Ever since I was a child, I knew that monogamy was not for me. Growing up, I always assumed that meant I would simply never get married, because who wants to be with just one person for the whole rest of your life? Blech.
I always knew about non-monogamy as an abstract concept, all that "free love" stuff from the 60s. I'd heard that some people lived in communes and rejected traditional marriage. I just didn't realize that there were "normal" people who agreed that monogamy wasn't all it's cracked up to be.
The best way to answer your question is that I never discovered the "concept" of polyamory, only the fact that there's a word for it and that I wasn't the first person to come up with the idea.
I don't need labels to define me. They're sticky and I hate the glue they leave behind.