Sorry to say this, but I sympathize with your partner. Mostly because I am in the same position right now and wish to have a Poly relationship. The only difference is I have a DADT policy, so my partner isn't really afraid of me sleeping with other people. We also don't really have sex, last time being 3 months ago, so of course I have natural desires and impulses.
I have connected with someone in the last 3 months and we have really developed a special relationship. It is so special, that I would like to openly add it to my existing relationship. But alas it looks like it will never come to be.
I often wonder if somebody really loved me, why would they deny me true happiness?