It was not something really presented as developed over time...
I was the only one with a prior experience with multiple partners at a time (MFM). My wife was (*GASP*) a virgin when we met. I only relieved her of that burden about a year before we were married. Before we even tied the knot, she would point out other women to me. I asked her why and she said "you only have one place to go with your excitement/frustration". Now this got me to thinking that she could possibly be okay with the idea of me and other partner as there was the pointing out of other women (and I did start pointing out other guys) and we had joked 'Not without his/her permission' on more than one occasion.
I brought up the idea after I had done a bit of research myself and we had many numerous discussions about our fantasies, likes and dislikes of the opposite sex, personalities of others that we liked/disliked. We also discussed what we felt the difference between sex and love was. (Yep, to us they are VERY different things)
The last thing I wanted to do was cheat on her and I felt that she deserved to have more than just me as a sexual partner. I too have always had a fairly large appetite for women... just can't seem to get enough. My wife says it's okay, I am a man after all.
So eventually, about two years ago, I signed up on this board and suggested she read along with me. She did for a while, but life began to distract us with various family/work/life issues. I came by from time to time, but mostly to lurk.
This year, things were going quite well for us and we began talking about it again. I told her my account was still active and why didn't she make one for us both to start gaining even more insight into this curiously fun sounding Lifestyle.
She agreed and began asking me more and more questions about how I would feel if she was with another man. She was truly worried about how she might be really jealous, so we discussed this issue as well.
After we made the decision to give it a try, we signed up for Swing Lifestyle and joined a few groups in our area... the rest as they say, is history...
Sorry for the ramble... I just wasn't sure a simple answer would be clear enough.