Looking at this from another angle: you wanted to be poly. He tried, for a long time, to give you what you wanted, he tried to be okay with it. And you now blame him for trying so hard and for so long to give you what you want and sacrifice himself for your happiness.
If he'd said, "I know myself, no, I'm not doing poly," what would have been said about him, either by you or other polys? That he's too close-minded to try? That he's trying to control you? That he thinks he's in charge of your body and your sexuality? That he's forcing ultimatums on you?
I think many mono spouses feel they have the choice of agree or lose their marriage. I think many mono spouses originally find a girlfriend/boyfriend because it feels too uneven and painful otherwise, but the men, especially, find that they're very often home alone with no girlfriend while their wives are out with a boyfriend. If they say, "This hurts and I don't like it," they are accused of all sorts of things, of control, of playing tit for tat, of jealousy, of not managing their emotions. So they say nothing, until they finally can't stand living like that anymore.
I know of one couple who used to be here--primarily the wife. She told us here how gloriously happy they were, how her husband was so wonderful in his acceptance of her boyfriend and poly-ness, etc. On the mono spouses of polys group elsewhere, I stumbled across her husband. He saw things very differently. He saw exactly the choice I describe above and had every intention of leaving the marriage as soon as the kids grew up.
I'm betting he had told his wife at least once or twice that he was not happy with her having a boyfriend, and he quickly learned that his feelings on the matter were not welcome. So he quit saying it and she chose to believe his feelings just magically disappeared.
As another example: I did this myself with my husband and his cheating. I told him a few times it better stop or I was filing. He lied, denied, and went on his merry cheating way. I quit wasting my breath, figuring it was his problem if he didn't believe me. He chose to believe I was fooled by his brilliant lies again, and lo and behold he was broadsided when I followed through and did exactly as I'd told him I would.
Again: this is exactly what I see in your husband's decision. Re-read my first three paragraphs.
Might he possibly feel the same about agreeing to a monogamous marriage and then having the ground rules changed? Yes, he may have 'agreed' to it, but....re-read my first three paragraphs above.