Listening to what's been said and really just have one point:
When exactly did he tell you he was poly? More specifically, before or after kids? Because kids are a whole other issue. And, yes, it's much harder to leave someone after that...you have more than yourself to consider.
If he told you AFTER you had kids, well, that's wonderful he came to his self-realization, but if you can't handle it and need monogamy, I'd say it's his job to suck it up and do what he needs to keep you in the relationship. You shouldn't have children, if you're still a child, and if he wasn't done growing up before he had a kid, it's his problem now.
On the other hand, if you came into the relationship knowing it was poly, hoping he'd change, thinking, maybe that having kids would make him want to just be with you.....then I'd say you knew what you were getting into and need to find a way to be happy, without making him unhappy.
I never quite understand people in relationships where one person's happiness is dependent on the other person sacrificing theirs...but sometimes the situation just happens