It could take years before a person is ready to be confronted with their partner having sex with someone else in front of them. You could very well be in shock.
Your feelings are valid. Don't be afraid to look at them, even if they are poly-negative.
While I agree with the first sentence, Sage, in that she could be in shock... I find that there is a tone of "you aren't poly unless you can pull off watching your partner have sex with someone else" in the rest of what you say, especially in the second sentence where you say that her feelings are poly-negative.
How is having a negative reaction to watching your partner have sex with another "poly negative?" What is "poly negative" anyway? (Actually, don't answer that, as I think it might be thread-worthy.) How is having strong feelings of this nature "negative" in any way?
Nautilus, I see no reason why you
need to be watching your partner have sex with others. If it didn't jive with you, then don't do it. Chalk this one up to experience and move on.
I personally am completely uninterested in watching any of my partners have sex with others. I have been in threesomes with my husband, PN, and our (former) shared partner, where I have watched. I didn't mind that, but I don't want to see him with a woman again. I don't want to watch my current gf Derby with her husband, or anyone else. I don't want to watch my current bf Mono with anyone, either. (Well, it would be a whole other ball game with him, but that's off-topic.)
I'm sorry this has caused you pain. I can relate entirely. I don't see anything wrong with how you feel. I am getting anxious just thinking about the times I have watched PN at swingers events. I have a cold sweat now. Yup, I get it...
It's not jealousy for me. In my experience, it's complete horror, I think because I know how sacred an act PN sees sex as now, and how bonding it is for him and me. Neither of us are into casual sex, and we learned this from experiences such as yours. It paralyzed me for a time. Yes, I think that was shock. It did him, in his own way. So now we just don't do those things anymore. Those feelings subsided and have gone, unless I get triggered, like I was from your post. We are better for having gone there, but are really glad to have figured that all out and moved on.
Perhaps something that happened around this event that caused this reaction. Perhaps your body is sending you as message about your nature and how you feel about sex. Perhaps you felt obliged in some way to watch, or to allow this threesome situation to occur, when you normally would have spoken up?
There could be many reasons to talk about this with your partner. I think you should explore every single one, even if you think that they might not agree or appreciate what you have to say. It sounds like it's very important to get to the bottom of what is going on for you before moving forward to more adventures in opening your relationship.