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 do 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 threadworthy. How is having strong feelings of this nature "negative" in anyway?
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 boyfriend where I have watched and didn't mind that, but I don't want to see him with a woman again. I don't want to watch Derby with her husband or anyone else and I don't want to watch Mono with anyone either... well he would be a whole other ball game of stuff really.
I'm sorry this has caused you duress. I can relate entirely and 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 to me, in my experience, its complete horror. I think because I know how sacred an act he sees sex now and how bonding it is for him and I. Both of us are not into casual sex and we learned this from experiences such as yours. It paralyzed me for a time and yes, I think that was shock. It also did him in his own way also and now we just don't do those things any more. The feelings subsided and have gone, unless triggered like your post did. We are better for having gone there, but are really glad to have figured that all out and have moved on.
Perhaps there was something that happened around this event that caused this reaction? Perhaps there is a message that your body is sending you as to your nature and how you feel about sex? Perhaps you felt obliged in some way to watch or allow the situation to occur when you would normally of spoken up?
There could be many reasons to talk about with your partner.... I think you should explore every single one, even if you think that they might not agree or appreciate what you 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 "open" adventures.