I agree with Sage -- your feelings are valid & ok to have. Now you need to work through the "why" you're having them. I told my partner that right here, right now, I don't think I'm ready to watch him make love to another woman. We've discussed 3somes & I said yes, but the other woman needs to be someone he doesn't love -- that sex is one thing but making love is different. He understands that & is respecting my feelings.
Then the compersion comes in -- I get that & know his making love to other women (like his wife) doesn't change his love for me, but I'm just not ready to actually watch it...and don't know if I ever will be!