I'm so glad to see you have moved on. I saw you posting a few weeks back elsewhere (Mish? K?) that sounded like you still hoped somewhere small that he would have a change of heart.. but clearly things have changed since then. Bravo for you. Seems these photos were 'the straw that broke the camels back'. Must have been hard to see.
This is very very sad, for Dan, I dont vilify him, I feel very sorry for the guy, as I said before about this stuff. He is hard-wired to behave this way. Something (who knows what) happened to him in childhood/adolescence and he is now stuck acting out these scenarios. I agree, he was probably going to post the pics on a s*x site.
Did you talk to your C about this yet? From what I understand, you were the ONLY person close to Dan who has found out about this behaviour (other than random strangers he interacts with in it).. you got to see behind the curtain, so he had to build it back up thicker and stronger. I am sure Dan feels huge loathing and deep rooted shame and KNOWS that how he behaves is wrong. He probably feels disgusting. Because you know the real him, you had to go, sadly.
I always thought these deep rooted issues are the cause of his leaving you, that he cant face himself in your reflection. Not that you arent good enough/pretty enough/a good wife, or that he doesnt love you.
I'm no pyschologist either, but I believe there is no cure for Dans predilictions, hence the term 'hard-wired'.
