Okay, thank you for clarifying. It all makes a lot more sense now. I see how the porn has been a real issue. It did interfere with your M and it was something you needed to change... but it does not negate that the A has been a long term betrayal of huge magnitude.
Even if you view porn as being as bad as adultery, having an A in response to adultery is completely unacceptable. People who have revenge A's usually regret them. You're WW didn't just have a revenge A with another man. She has been living with you while involved with another married man for (15) years. There was no regret until the game was over.
I can see how your guilt over porn would make you feel like you deserved to be with someone as damaged as your wife, as if perhaps you deserve each other but that's only true if you are equally committed to becoming better people. Proving you can obtain from porn will be a lot easier than your wife proving she can be trusted.
On whether or not porn itself is a big deal... Or how big a deal it is... It depends. Hard core, soft core, a fleeting scene in an otherwise innocent movie, an obsession... It depends. I'm not personally a fan, most of what little I've seen was icky. I think the Hollywood image of women's bodies is damaging to both men and women because none of us are sure what normal is anymore and often, we don't celebrate our bodies unless they fit the Hollywood image. But, that's not just porn, it's most of the entertainment industry.
Other than that, I think it's like eating empty calories or consuming alcohol. It may not be good for us but indulging in a guilty pleasures once in awhile isn't going to hurt. When you continue consuming even when it's creating problems, it's probably best to eliminate it from your diet. And there are some people who really just can't indulge, even a little, without it creating problems. Only you know where you fall on that scale.
Meanwhile, as I said before, even if your porn viewing was on the same level as an A... Having an A in response would have been inappropriate. Having a 15 year relationship with another man while completely screwing with your head was completely unacceptable. You did not deserve that. If you were doing, and refused to stop, something she couldn't live with, she should have gotten a divorce. She stole those years from you. She stole your ability to trust. She made you feel you deserved to be treated that way. Nobody deserves to be treated that way.
Good luck with deciding how to proceed. What ever you decide, if there is a God worth believing in, they would not judge you for not being able to forgive so much betrayal from someone you can no longer trust. If you are to stay married, WW will have a lot of work to earn your trust on a daily basis. If she's up to the challenge and if you aren't ready to move on to a different life, I wish you both much luck.
There's a book called, "How Can I Forgive You." If you haven't read it yet, I highly recommend it. It talks about the difference between forgiveness and acceptance. As someone who's been betrayed, and as a pastor, you might find it very helpful to you and those who come to you for counsel.