I'm going to go against the grain here and many will disagree, but men are sexual beings, whether us women want to accept that or not. The porn industry is one of the most profitable. And it's not women buying it!
Men need sex and when they don't get it from their wives, they go elswhere. Is it right? NO, but to them, it is!
The #1 reason why married men cheat: lack of sex (to them love) at home.
Men feel a huge rejection when the women they love doesn't give them sex. They equate sex to love, like women equate emotionalsharing with love.
Not saying his disgusting cheating was in ANY way you're fault, but like you said, there were many issues in the relationship and well, he was not mature or well equipped to handle them.
By the way, I know 2 couples in "healthy, non addict" relationships where one of the partners is cheating and the reason, No sex! It's not just about addicts, it's about broken spirit and deflated ego.