I chose "other."
The way I became non-monogamous was banal, unconventional, and romantic. I called off the wedding with my fiancé Red, moved out, but kept the relationship going. I wasn't looking, but then met the man who would become my husband David, and I started a relationship with him. A woman with two men going at once, not that unusual, I suppose.
Since Red and I were so close, I immediately went to his apartment (what used to be our apartment) after the first time i had sex with David. We then made love too.
David knew about Red, and figured out quickly that it wasn't over between Red and me. Instead of dumping me, he said it was fine, and that he loved me. Shortly thereafter they met, and got along well despite being quite a bit different. One Saturday afternoon we were all together and we had the first threesome for each of us.
We mostly kept the sex separate, but with regular weekend threesomes. After a couple of months, I moved in with David and several months thereafter David asked me to marry him with the explicit promise that I could keep Red.
So on one hand one could say in the first year our swinging career ramped up quickly into the hundreds, while OTOH, one could say that I was just a slutty single girl. But David and I got married and I'm still sleeping with Red.