I saw a bit of the RedPepper's Journey thread and thought I'd borrow the title format :-)...
I decided I was polyamorous when I was 20. What instigated it was that I liked 2 sisters in Mexico and I couldn't decide who I should ask out. To top it off, I still wanted to create a relationship with a girl that I'd known 4 years ago back in Canada even though we'd never really been more than classmates (the life of the angst ridden youth -.-). It seemed that both of the sisters wanted to be with me but they were monogamous minded, so I had to choose; and I was afraid that if I chose one over the other that it wouldn't work out. Anyway, I ended up not being with either of them; while my family did have a house in Mexico at the time, we were only there for the summer vacations, so that's where it ended. But ever since then, I've decided that I should never have to choose between more then one beautiful women who likes me unless they force me to do so. Conversely, I don't want to make any partner of mine not see someone else that they're interested in. That being said, I do have one rule, that goes both ways; if I have a partner, and either of us dislike someone else that they're with strongly, we can give each other an ultimatum; either leave the other person or our relationship will end.
In all honesty, I haven't had all that much experience with polyamorous relationships. I had a bit of it with my last girlfriend; soon after I met her, I told her that I was polyamorous and she could take it or leave it. We were together for a year, but I think that ultimately, we just weren't compatible with each other. Nevertheless, during that year, I did have relationships with other women, albeit fleetingly; nothing all that physical. The most I did with another women was a few kisses on the cheek, but it was more then enough to make me love polyamory. Now, I no longer talk to my ex girlfriend, but I still talk to the girl I gave some kisses to, even though she's now married. That's life for you :-).