I think I had two crystalized experiences that "prepared" me for the idea of polyamory. One was indirectly due to my LDS (Mormon) background. Some of my family/friends buddies were having various debates/discussions by email. This was in the mid to late 90's. We were discussing the problems of the church's past with polygamy, when it struck me that polygamy might be okay if it wasn't just a husband with multiple wives, but could also be a wife with multiple husbands (or multiple wives and multiple husbands, what I believe polyamorists often call "group marriage"). Oddly, no one replied to my observation there, but the observation stuck with me and had effects later on.
Another time, probably not long after (a few years?), my wife and I were on an outing (of the mountainous kind, I think), and my best friend from grade school was with us. This friend was rather socially awkward, and the chances of him even trying to date let alone succeeding were rather slim. I had a mini-epiphany hit me while we were hiking along, where I wondered, "I wonder, what if there was some way I could share my wife with this friend, would I be willing to do that?" I surprised myself by realizing that not only would I be willing, I'd be quite pleased to do so. He was kind of a lonely guy, and I thought he was very deserving of the kind of love and company I had with my wife.
These thoughts were brief and fleeting, and soon displaced by other thoughts. But somehow they stayed with me over the years. And as 1995 crept away and 2005 approached, I found myself mentally rebelling against many of society's norms. LOL, I was in a suitably rebellious state of mind when I fell in love with a friend who was already married. This falling in love was mutual, and we both had the dilemma to face of, what do you do when you fall in love but one of you is already married. She did a lot of web research, and it was actually she who discovered the word and concept "polyamory" (and polyfidelity). By that time, I was quite ready to accept the idea, and she spent the next year talking to her husband about it. Eventually he accepted it as well, but I was already at the point of acceptance the whole time.
That's a rather short version of a rather long story, but you get the idea.
Sometimes we have independent thoughts that are like seeds in our minds. The seeds may sprout and grow many years later.