My husband actually wanted more of an open relationship at first but he realized that I am not wired that way. I prefer to get naked with people I know and trust which is why poly works for me. He prefers to be free to play at will (within a few boundaries). We decided to give our special brand of poly/open a try and we've been happy with the results.
Broached the subject initially through a bare all porn expose to see if she was into the same stuff as me. She didn't care for much of the BDSM/whips and whatnot play but was really interested in the cuckolding/humiliation videos. We've been fetishizing it and discussing it ever since.
My SO and I met and had a discussion about what kind of relationship each of us wanted without really thinking that the other may fit our desires. After a few dates, and many discussions we realized we wanted the same - a non-monogamous relationship with some bondaries. These boundaries have evolved over the past couple of months and will probably continue to do so, but we had our discussion from the begining. The one thing we agreed on from the start is to be complete open and honest with each other...this is crucial for this type of relationship and neither of us had that in our prior (mostly monogamous) relationships.