I assumed in high school that I would never get married, but I assumed I'd be a serial monogamist (not that I knew that term at the time!), ending one relationship when I got interested in the next person like I had been doing so far.
I read Heinlein in my 20's, but by that point I'd already met MC and established various non-monogamy boundaries (again, didn't know the terms. I just knew I wasn't going to tolerate jealous behavior and I knew I was a flirt). And to me, Heinlein was just another fiction story. I didn't really read it as a, "Hey, this is how it COULD be!" kind of thing. I was still trying to wrap my head around wanting to be with ONE person for the rest of my life (MC), let alone more than one, but I suppose it did help plant the possibility in my mind.
It wasn't until my 30's that my cyber-flirting with TGIB turned into something more and we decided to start using the terms boyfriend and girlfriend. Even then, it was another year or so until I learned the word "polyamory" and found this forum.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack