Kamala, I relate to a ton of what you're saying. I used to think I was primarily interested in sexual openness with the possibility of something evolving into more of a relationship under the right circumstances, but since winding up in a relationship with my boyfriend I've realized that I'm far more interested in actual poly than just being open. My husband, on the other hand, still isn't entirely sure what he wants out of additional relationships, and I've made a commitment to be supportive of him as he figures out what makes him happy, even if for him that means more casual sex.
He had a pretty casual hook-up with one woman, and while it was a little emotionally complicated for me, I pretty much handled it fine and was glad to have that "first time" over with. But when he recently developed a new interest, and I was getting the impression that it was again primarily sexual, I started to feel really uncomfortable with the idea of him just having a string of hook-ups. I talked to him about the possibility of trying to take things a bit slower sexually, but he wasn't happy with agreeing to that. It lead to a whole lot of talking, though, and the end result was more or less that I made it known that I would be *most* comfortable with things that move a bit slower sexually, but my husband is still free to make his own choices, and if things happen in a way that fall short of my ideal comfort, he knows that I'll need a bit of extra reassurance from him and he's happy to give it.
We try, as often as possible, to re-frame things in a way that's about *our* relationship. So rather than "Don't do X with Y," we try to turn it into "I need Z from you." Making it about asking to have our needs met from each other rather than criticizing each other's behavior with other partners seems to keep things in a really good perspective. So I guess my point is, are there things you can ask your partner for that would help you feel more comfortable that aren't about "don't do this with her" but are instead about "help me feel more comfortable by doing/showing more of ____ with me"?
In our current situation, what I ended up needing more of wasn't even so much about me. I told him that it would make me feel better if he would talk more openly about the non-sexual ways he's interested in this girl. Since then, having conversations and seeing how excited he is to be embarking on something new in a way that isn't strictly about "I'd tap that" has been really fun for me, and I've been able to feel really, genuinely happy for him. I'm sure I'll still have my moments when/if they actually have sex. But it's made a huge difference for me to figure out ways that he can help me to feel more comfortable.
Sorry this is so long and rambly, I just feel like I really get how you're feeling!