As always with this subject, there is a large number of misunderstandings and misconceptions.
It's probably not true that all men watch porn. Most do though.
Most importantly, porn watching - and male sex drive generally - is not fundamentally rational. The idea that you can create a simple chain of cause and effect from TELLING him that porn is a deal breaker, to him AGREEING to stop in order to make you happy, to him NEVER looking at porn again through sheer force of WILL, is ridiculous. It doesn't work that way.
Sex, for men, is a direct, unconscious motivational force stronger that any other force we experience. Most men if the opportunities were available and there weren't balancing negative consequences, would shag pretty much anything that moves. Porn offers a way to effectively do that (as far as the brain is concerned) without consequences like having to manage a relationship with the other person, STDs, expense, or (as the user sees it) committing infidelity that could break up relationships and families.
When the OPs DH said he would never look at it again, he may well have meant it perfectly honestly. Or he may not. The problem is it's an environmental thing. While porn is so easily available in the environment, and the worst consequence of using it is having to keep it secret, male brains will gravitate to it, no matter what kinds of well meaning and earnest conversations happen at other times.
I don't know where that leaves women who see it as a dealbreaker. I can see it's very difficult. You could not have an internet connection in the house, but he'll still see it elsewhere.