I remember being confused why both of the adorable 1st grade boys who were fighting over who had kissed me on the cheek couldn't both be my boyfriends.
I read a lot, including a lot of sci fi and fantasy so I was reading Heinlein, McCaffery, and others with non monogamy at a young age.
When I figured out I was bisexual I decided I wanted both a wife and a husband. Since I had also figured out I was something of a submissive switch (bdsm) I decided I wanted a triad with a male dom and a female sub. (Therefore I drew a lot of inspiration from religious polygamy, since that's the closest model I could find.)
When I had my first "serious" relationship with a male I encouraged him to date another girl he found interesting, even though she wasn't interested in me too.
The entire time of my marriage I've encouraged my husband to play with other women if he was interested. For many years I was still in love with my ex, but I was forced to chose.
I didn't actually start researching the term polyamory (and therefore figuring out there are a LOT more configurations than I had previously understood, particularly the idea that I might actually be able to have more than one loving man in my life) until a few years ago. Since then I've deliberately pushed towards a full fledged polyamory model.
Currently, I'm working through the kinks (no pun intended lol) in my first actual relationship with another man that is concurrent with my relationship with one man.
I've always known I had the capacity to love more than one at a time. It doesn't make sense to me, otherwise. I could easily see myself as eventually being the happy slut who loves and plays with many, if it weren't for my habit of attracting D types who forbid it.
So - yup - without knowing the terms, I was poly before I "had" to be.
Guy - Dom/husband
Minx - sort of dating
Cru - (no longer dating)
Lee - Guy's girlfriend