Update, reviving this thread. Thanks for your replies and thoughts, everyone. I was so confused before. One more try at talking this out, and it turned out WAY clearer than i'd anticipated! My husband and I had a LONG talk today and I found out:
1) He's thought we were polyamorous our entire marriage but we just hadn't found anyone yet, and weren't mentioning it because we were having so much fun together. When I asked "are we monogamous until further notice?" and he said yes he actually meant in his mind, we were philosophically polyamorous (as we both believed and agreed polyamory was the right path when we got married). I'd originally thoguht "monogamous until further notice" meant just that - we were monogamous and when one or both felt like changing to poly we would talk about it. NOW it all makes way more sense to me... for whatever silly reason. Changing from monogamy to polyamory because of a single other person really hurt my feelings but now that I know that wasn't his intention it feels WAY less painful. (I've also been working on my self esteem and other issues)
2) He still considers us to be polyamorous, but was saying "We're monogamous" to not hurt me any further, since he'd decided that sex with the other person wasn't worth hurting me.
3) He believes in my freedom and thinks I should have other partners if I want (ideally after we talk and he meets the person).
4) He doesn't really think rules or restrictions are ok in a polyamorous situation except for the "ideally..." part above.
Well now I feel a humongous sense of relief. The forced monogamy thing never felt real or right and now I know he was doing/saying those things because he didn't want to hurt me any more (just like Cieldumatin said). It makes sense, makes me feel safer and loved unlike the confusion of thinking he'd changed his mind just -like - that.
Now I know we have a long way to go... he has to be ok with being honest with me and not afraid of me "flipping out", and I have to not flip out and trust the living heck out of him. And be honestetc
-to be continued hopefully