I'm not officially at work today, but I wanted to weigh in on this one. Oh boy, do I know this one from a personal perspective (and I totally understand your anger and confusion her, I have so been there), but that's also because I know about it from work, too.
Generally, I think it's safe to say that the long story short of this as a thing someone might do is that wanting to be poly -- to be open, honest, to have all of this be consensual -- is one thing. That's just not what someone else wants who wants to be with other people, though. And by all means, wanting to have other partners for yourself doesn't mean you want your partner to have the same privilege or right.
In a case like this, where this person was very clearly, in many ways, seeking to control you, he would keep you from that and many things because of control. He would also lie about his doing what he was doing because this person does not have regard for you: no one has real regard for us who abuses us. They just don't, as painful a truth as that often is to deal with.
He lied and kept secrets because those were excellent vehicles and tools for his control. He did what he want without letting you do what you wanted, because letting you do what you want means not having that control over you. Considering your real needs would require he had a real regard for you when he didn't. He didn't want to be monogamous. Obviously. That was a lie. He told you you did to control you and to keep in line with the part he was playing.
All the answers to all the questions in this in this case really all likely just boil down to this: he did what he did because he was emotionally abusing and controlling you.
I want to remind you again, because I don't think it can be said enough: none of this is your fault. This man's behaviour is 100% his responsibility. Not yours.
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead