Sometimes there are hard lines.

You laid out the opposite boundary to him first. You said you weren't going to change on this. Now he's saying he doesn't accept that.

Either you two find some kind of compromise or you walk away. There isn't a third option. The two of you want contradictory things and have clearly expressed them.

My only advice is you both sit down, be calm and non-judgemental and talk about your feelings towards porn. Don't argue with each other. Try to understand each other. It may be in the process of coming to understand why each of you feels the way you do that one of you softens their stance. Maybe not. Don't make it an argument. Don't try to debate each other. Sincerely make an effort to understand and see if there is any change of heart.

Otherwise, walking away is the only option. You can't both have two contradictory things.