I think I knew I wasn't "normal" when my fiance in college "cheated" on me by having sex with another woman. I knew I was "supposed to be" upset by this, and I guess I did a credible job of it, but my heart wasn't in it. I wondered at that time (all of 19 years old, mind you) if it was just that I didn't love the guy like I thought I did. It wasn't until a year or so later that I began to get to the truth of it.
I was expecting our first child when my husband (another man, not the one I was just talking about) confessed to me his attraction to another woman. We'd tried hard to base our marriage on honesty, and so even though I was as big as a whale and feeling anything
but attractive, we talked it out and decided to "open" our marriage. We didn't have the word "polyamory" then (this was 1983), we were just trying to do the best we could for each other. We'd read Heinlein's fictional works that included these kinds of polymorphous, open relationships and found them intriguing -- could we do the same?
Well no. That was overly idealistic, as it turns out. The number of people capable of functioning at that level of heart-openness are vanishingly small -- and neither my first husband nor I always lived up to our highest ideals, I must confess. We just kept trying, right up to the point where we couldn't be together anymore.
It wasn't polyamory that ended my first marriage, though poly always shows where the cracks in any relationship are located. My second marriage was predicated on polyamory from the beginning and has remained so to this day, even though we're more interested in "friends with benefits" than long-term romances -- this feels more like "swinging" to us, though I realize that not all swingers feel comfortable forming these kinds of friendships...
Anyway, that's the short version of how it all started out for me.
I think I'd enjoy moving more and more away from the labels for these things, if I could find others who were interested in doing the same...