It isn't at all uncommon to consciously believe or choose X, Y or Z, and yet still have some part or aspect of one's self left out of that conscious belief or choice. How or why it works this way, I don't know, but I do know that some "parts" of me are always playing catch up with other "parts" of me.
So it may be that you're consciously quite capable of warmly giving your blessing to your partner as she is intimate or loving with others ... yet some "part" of you isn't yet so evolved or comfortable as the rest of you. And so there's probably some spelunking to do.
I'd recommend looking very closely, carefully, with refined vision and delicate attention to find out and name the precise nature of the fear some "part" of you experiences when challenged as you describe above. Is it fear of abandonment and loss? Is it a fear that the love you share with your partner may be made less by spreading it out among others? Find out. Only you can do that. Whether through dream analysis, astrology, tea leaves, i-ching or phrenology, ... whatever brings you to that "Aha! That's it!". Often, our submerged fears or worries disipate when we can know them, feel them, and name them.