Stoneycase completely hit the nail on the head. He feels one-upped, outdone. He's made it clear that it's not the actual threesome that's a big deal, it's exactly as you said, he just doesn't see it as being "fair" that I've done this outlandish thing that he never has. And, as you said, I did tell him once that I didn't think I could handle it and he very politely told me that was ok and that he would never want to hurt me...but, also like you said, the underlying issue remained and reared its ugly head later: he is still jealous of my prior experience.
Thank you all so much for your responses. Several of you have made good suggestions. Stoneycase, you're right that coming up with something "creative" that is new to both of us may be a good way to get past this. Maybe another outlandish but different experience is just what we need. That's a great idea and may be just the solution.
Goose, you bring up a good point too. The truth is I don't completely understand my issues about it beyond "the idea of him with another girl". I'm actually not afraid at all that he'll leave me for whoever we bring home together. He's made it clear that he doesn't even care if it’s one of my really unattractive friends...he just wants the experience so he doesn't feel one-upped anymore. But for some reason the thought of seeing him kiss, touch, do ANYTHING with another girl just turns my stomach in knots and makes me feel like I want to throw up and cry at the same time. So why do I have such a problem with it? I don't know. I think about having a threesome with him and another man and I can’t bear the thought of touching another man, let alone in front of my SO. I guess I just want him to feel the same way. I want what I can’t have…a man that wants me…just me…only me, and doesn’t even want to touch other women.