I "came out" as poly to my ex-girlfriend last night. It happened to come up in conversation, so it wasn't like a "I have something very important to tell you" thing.
The first thing she asked was whether I get tested for STDs, which for me is a "Yeah, duh" but it's a legitimate concern, I suppose.
The next thing she asked was whether I ever get jealous. I told her every once in a while, but for the most part both Master and I are fine - and when one of us does get jealous, we talk it over and work things out.
"Man, I don't think I could do that," she said.
"It's not for everyone," I replied. So far, so good. Pretty standard stuff, and she's taking it well.
"I dunno - that doesn't seem like a real relationship to me. You guys aren't making an actual commitment or anything."
I pointed out that, ideally, one enters a relationship for many more reasons than just sex, and that Master and I will always hold the relationship between us as our first priority. (Even then, some poly people don't prioritize their relationships, which is still a valid form of doing things, but I thought pointing out this aspect might appeal to her monogamous values.) She didn't buy it.
It's possible that part of the problem was that I wasn't accurately describing the nature of mine and Master's relationship and she wasn't getting the right impression; I was working on homework at the time and didn't have the time to hash out the nitty gritty details of my sex life. I also didn't mention the kinky part of our relationship; on the one hand, it's probably a good thing I didn't, considering her moderate-conservative streak, and on the other hand it does make the poly thing easier to understand, I think. We may play with other people, but he is my only Master; I am his only pet. That's what makes our relationship significant (among other things).
I was tempted to point out that she has little to no authority to criticize the validity of mine (my?) and Master's relationship, since she has since "gone straight." This may just be me, but saying she no longer dates girls feels like she's invalidating our two-year relationship in high school. But that would have been petty, and probably would have just pissed her off.
*sigh* Oh well. Can't win 'em all, I guess.