Some of us are born poly and bi. I've found I can force myself to be monogamous for the sake of the man for about two years before I can't take it any more. If you've lost interest in sex with your primary, that's a big red flag that your relationship's in trouble. Have you let him know it's serious enough to consider a counselor? If you consider getting one, be sure to find a poly and gay friendly professional before you bring up the subject with your guy. The last thing you need is paying a closed minded psychologist incapable of grasping your situation. I saw a jerk once, who, when I told him I wanted an open relationship, instantly decided I was a hysterical personality.
It's going down hill any way you slice it. Either you'll stay together and you'll stay quiet about your needs and you'll burst open at times in private and find a partner and keep it to yourself, or worse, he'll find out, or you'll split up in a messy way b/c you never were able to talk about it. Regardless of what he says or does with his emotions, you have needs and you should be able to discuss them openly, and you should be open to hear his response. If you can't do it just the two of you, find a therapist, either a couples therapist or a sex therapist. It can take time to convince him to join you in therapy, and you'll have to tell him why at some level, which means starting the conversation, but honesty is your best chance. In the meantime, read Jenny Block's "Open", as she has similar challenges, and if you can get him to read it, all the better! It's short, and comforting. Best of luck.