In the first year of my current relationship, I walked into the room and found her kissing another guy. We were having a party and were all drinking alot. They were standing in the doorway. I walked up and slammed the door on her ass knocking her outside. I then locked the door.
Eventualy I forgave her. A few months later, my suspicions of her came back. So I too "invaded" her privacy. I did the exact same thing that you did. I archived her IMs. I found the same thing, you did.
She tried to turn it around on me, and make me out to be the bad guy by "invading" her privacy. Eventualy, I forgave her again and we moved on.
Today, we have the most trusting relationship. I would never snoop into her private areas of life. And she wouldn't cheat.
The point is that by my investigating the matter, we were able to find the problems and deal with them. Had I not looked into it, maybe we wouldn't be together today?
On the other hand, if the man has the stamina to keep his wife satisfied, and continue to look at porn, more power to him
Call me Stamina Man!