I agree with Newt, if i discovered my DH doing this behind my back i would absolutely go made at him. Cheating in my mind is not neccessarily physical i would feel more betrayed by an 'internet love affair' (secret messages and emails etc) than i would a one night stand. I personally believe it is more of a betrayal to write such personal intimate things, whether it is on the internet or simply emailing someone at work.
You are totally within your right to feel upset with him, he needs to understand that desiring another woman and contacting her to sustain this 'desire' is not acceptable, in my mind (not sure whether many people will agree) but i would class as cheating. I'm not very good in explaining what i mean, but hun you need to maybe write him a letter if he won't talk to you.
Just explain how it makes you feel and then leave it to him to make the next move, he is being immature in not discussing it and obviously feels like he is doing something wrong otherwise why hide the screen and change the passwords? guilty consience i think.
I hope you get it sorted soon, try not to worry i think most relationships go through similar things it makes them stronger in the end.
