But having said that this is a terrible place for advice in this situation, I will of course still add my two penn'orth... (don't worry, the irony is not lost on me)
Firstly, your nqsDH clearly outed himself. It's a curious thing that men (and some women) do when they've got themselves into something over a long time and would like to get out of but don't have the mental means to do so. Subconsciously they deliberately drive the ship onto the rocks, safe in the knowledge that they will either be released by drowning or someone will save them. You said yourself that you're not a snooper - after 12 years of being careful, he left his email wide open. He wanted (consciously or subconsciously) to bring this to a head. The logical thing would have been to say to himself "I've had enough of this now, I'm going to delete all my accounts and try to reconnect with my wife" - but that's not usually how life works.
Nobody here seems to understand confession. 'Confession is good for the soul', so the saying goes, and it's pretty much true. By confessing all, your nqsDH has taken all the anxiety and stress off his shoulders and plonked it down onto yours. He's flung himself into the sea and is now waiting to either drown or for you to save him.
I don't know whether you even still like him enough to want to stay married, that's something you need to decide, ideally with the help of real counselling.
There's just one factor which worries me here
You seem to take an unrealistically hard line on porn.
All men like watching porn.
Every. Single. one. of. us.
Ever since we were first able to daub pigment on the walls of caves or fashion figurines out of animal horn.
For most of us (I hope), porn is just something we passively consume, like food, air, sports, action movies and beer. Obviously your nqsDH has gone beyond that, into the interactive realm, which is dangerous and wrong - but maybe fixable.
You offered to watch some porn with him - this was a constructive suggestion (and I'm sure many men would love it), but clearly he knows your real views on porn so I'm not surprised he declined. I would too.
For most men watching porn is a shameful, guilty pleasure - in my case alongside liking fifties musicals and (worst of all) liking 'Sex and the City'. It's something we want to do alone.
I fear that if you can really not stand the idea of a man you're in a relationship with watching porn, then either you need to either get some cats or take an interest in space travel, because you're not going to find a man that measures up on this planet.
Anyway, must go, time for football....