Huh. I always thought polyamory was just a new word for what used to be called an "open marriage." Learn something new every day.
I am starting to feel better about this, I think... examining my feelings about her, I'm pretty sure that I feel the same way that I feel about my other close friends -- they are all very special and important to me, and spending time with them always makes me feel warm and happy.
I did hear something the other day, though, which sort of clarified the risks of the situation. One of my friends is in a sexually open relationship with a woman at her college. Over the summer, both of them slept with other people. My friend made a new "friend with benefits" with no romance involved; her GF met someone and apparently fell in love, and would now rather spend time daydreaming about the other girl than being with her girlfriend. I would never want my husband to feel the way my friend feels -- like she can't compete even though she's there and the other girl isn't. What I think I have decided, though, is that I would never put my husband in that position -- no matter what happens, I know that being with him makes me happy, and that he's really the only one I need.