Well, you say you "caught him" watching p 0 r n. And he's telling you he wants to stop doing it. So clearly there was an element of shame that he felt with you catching him watching p 0 r n.
My wife has also found me watching p 0 r n at times in our marriage and simply pretended not to notice. I felt no shame whatsoever - but wouldn't have even if she made an issue out of it.
We all need companionship in marriage, but we all need our personal space too. And one reason I'm sure we've lasted 20 years is that we both give each other space and respect for our solitary thoughts, deeds and actions. And that's so hard to come by that you'd never WANT to leave a relationship like that.
All guys (and gals) self service even if they're in a sexually fulfilling relationship. It's going to happen whether you know, like it or not. It's no reflection on how much he's into you. Maybe you had a fight or words that left a bitter taste but he's feeling like a release. Would you rather he self service or visit the neighbor?