OP, I hope you are doing ok. An absolute headfuck. Has he agreed to talk to you about it yet?
I think the way it stands at the moment, if you want to know the details of it all, then he should be giving them to you. he doesn't get to drop this bombshell and then say to you that its none of your business and he doesn't want to talk about it.
I can't imagine what I'd be feeling if I was in the same boat as you, but I know I would feel absolutely crushed.
The fact he knows your self esteem isn't the greatest at the moment and he did that absolutely leaves his excuse of he didn't think you'd mind as an absolute lie. Its not rocket science.
I don't know what to advise really, except listen to how you feel and know its ok to feel however you do about this. And its all on him. He has done this to you, his stupid pathetic actions had consequences, and those consequences are hurting the person he loves in the most crushing way possible. I bet it would of be far less painful if he'd just punched you in the stomach.
And don't let him make you think for one second you are over reacting, over senstive about it etc and whatever else he will likely come up with to try and make you shut up about it.
At the very least its going to be something that affects you for a very long time. For him, it might have been a stupid fleeting thing that meant nothing and he'll forget about it in a couple of weeks. But I think you need to make him realise that for you, its not something that is going away anytime soon, if ever.
He has royally fucked up. If he doesn't own that and give you exactly what YOU need right now, whatever that may be, he is adding to your pain. He needs to know this! Perhaps needs it spelling out to him - as he is apparntly so thick as to assume you'd of been ok with it.