That does sound frustrating. PolyCYNM is still right though, you two need to sit down and talk. Though it sounds like you've already told each other what you wanted - only you were told "no, you can't have what you want".
I'm not sure what to recommend beyond making sure that you are completely unambiguous about your desires. After that, if you are still required to be monogamous and you want to be otherwise - it sounds like a change is in order. At least that's my personal take.