Thanks for your responses! Some thoughts/updates...
I wouldn't say I have had enough time as a "swinger" to have the strictly-sexual aspect of that lifestyle programmed into me. In fact, I would say that is why I have found myself dissatisfied with it. I also think that, when you start "dating" (I guess by which I mean exploring a beyond-platonic relationship with anyone), you don't know yet if it is going to evolve into love or if it will move past a a certain level of connection. So I am slightly skeptical about the clear cut difference between the "swinging" mode (ie sexual approach) and the "poly" mode (ie loving approach). I think all non-platonic relationships fall on a spectrum between love and sexual connection. I mean, when I think about all of the serious, emotionally committed relationships I have had - they often started off as a more straightforward instinct of attraction. That is what made the relationships move past friendship, the sexual element. And sometimes it fizzled out, and the emotional depth didn't develop, and sometimes it became very deep and serious. But its hard to start off any relationship with the knowledge of "I want to love you" versus "I just want to fuck you". I start with an instinct, and follow it to find out what the potential of that particular relationship is.
ANYHOW, I have a real update on my story! My primary partner had been away for work for many weeks (with a few weekend visits in there), and this past weekend, knowing I was going to be seeing this friend who I have been developing this connection to, he encouraged me to make a move with him. So I did! We had a really great night together, that felt really open and honest and passionate and intimate, though still taking things somewhat slowly. Unfortunately, I am going to be away for quite some time on a job, so we have to put this on hold until I return. My boyfriend and I have been talking about it, and he is comfortable with me pursuing this more when I return home. I want to take it slow, and am sort of glad we have this time apart to think things over and not get totally overwhelmed by these feelings now that we are able to act on them.
Phew! Any new wisdom for me now that I have taken one step down this path? It has been really helpful to get my mind working in new ways reading your past responses.