Another nut job here lol
But seriously, sorry that you're disappointed how it went, but to be honest, IME and from what I've read on here, they very rarely if ever hold there hands up straight away and say yes, you're absolutely right, I've been giving an ow loads of attention and sex and I now realise that's its very wrong and I will stop it straight away. Please forgive me and you all live happily ever after...
No, I've never seen that happen.
Somewhere unthread someone mention that his anger is fear and I'm inclined to agree with this.
If he thinks its all a bit of a laugh, ask him why you were excluded from that particular bit of fun. Why did it have to be a secret?
Oh because you wouldn't have liked it. Funny that......
You may have to have more of these conversations if you want him to come clean and admit an EA or whatever.
From reading what you've said, its almost like its not what he's actually done, but the way he is dealing with this that is causing you more heartache.
I also like another posters comment of showing the ow DH the screen shots, see if they get the joke
I don't know if I would actually do this, but I would probably threaten it.
When I suspected my DH of being economical with the truth one of my comments was, well shall we ring her then, see what she says, and that would make him sit up because he knew that I would!
Once my DH knew I'd seen secret email accounts he didn't try and deny any more. He did delete, but not all of it went and I recovered some of it. What he was economical with was the extent of it. Whether he'd met someone once or perhaps twice for example. Which in the mix of things may not be important but to me it was.
Can u see if he's messaged her again recently..? This doesn't necessarily need to be the end you know, but to move forward he must be truthful and not minimise how hurt you are by all this.
Sorry if I've rambled on 
Keep getting interrupted!