I cheated on my partner online and he cheated on me before also online. For us, it was cheating but as Open Eyes says, many men and I would also dare to say women don’t see it like cheating.
In these cases, from my point of view the best is how you both see the matter. Why he wasn’t honest when you both made a deal about deleting the app. If it’s still worthy to go on with the relationship or not, working on the relationship to improve it...

You are the one who knows better your husband. You have the right to know the truth.
Do you think talking straight away with him is gonna be effective? Or do you think make a little of research before about what’s he doing in the app is gonna be better? You are the only one who can reply to these questions.

What is normal and anormal in a couple is only a matter of them. How they both consider what’s happening. So, I wouldn’t worried about the porn stuff unless it’s an addiction.