I kind of fell into it. I have been mono all my life until about 1.5 years ago. I'd never thought about living out a poly relationship until I married my current husband 3 years ago. He wasn't poly, but he is bisexual. I always felt like asking him to never be with a man again because we were now married was selfish.
At first all we did was talk about it and we pretty much dismissed the idea. But he made friends with a co-worker who is poly and quite out and open about it. What seemed like a really weird arrangement to me at first kind of became less weird over time just from hearing about their life through my husband.
About 2 years ago, I decided 'to hell with it, let's try opening things up'. Why? Because I had noticed that my husband seemed to get more antsy and talked about his desires of being with men more and I could see him struggling. At first, it was him that sought out a male FWB. In time, that friend became my lover and he and I fell in love. I now refer to him as my boyfriend. I never expected this but I'm glad it happened...
I tried being reasonable. I didn't like it. ~Clint Eastwood~