Thanks for your thoughts on this, this site has helped me quite a bit in my past few days of mulling this over.
I think what was lost on me about polyamorists is that they truly are able to love more than one person, not just sleep with multiple people. When I feel a loving connection growing with her, it's not necessarily cheapened by the fact that I'm not the only one.
It was easy to feel that if I wasn't her only love or at least classed in some "primary" way, that what we were building was just an extended f-buddy relationship, but I was confused by how open, trusting and supportive she's been, if I'm just a "piece on the side."
I also hadn't given much credence to how hurt she might be if I just continued, planning to skip out when I found another person to be mono with. Of course, if she really is loving me, then losing me will sting like it normally would, rather than just "oh, well, there goes one of my lovers. Ho hum." So I need to be honest with both her and myself.
One of my lingering worries is, my circle of friends are mostly married, or committed. They want me to have a wonderful partner, and those who know about this aren't thrilled to hear I'm with someone who "can't make up their mind", or doesn't value me they way they think she should.
I wonder how to make it clear to people that I treat this relationship like something "real", without seeming like a weak, cuckolded guy.
Thanks again for opening my mind to the possibilities of the situation. I hope to go forward with a healthier viewpoint.