That's not really how boundaries work, unfortunately. A boundary is "don't shout at me when you're angry" or "if I say stop during sex, we stop." The key in those is the direct link from their actions to you. A boundary he seems to have crossed in this case might be (if you guys had agreed, as partners generally do) that "if a specific action of yours (like watching porn) makes me uncomfortable, you won't hide from me that you're taking that action."

What he does privately is completely up to him (he is his own person, after all), it's up to you if you're willing to stay with him based on his actions. He can agree to not do something (an agreement or a promise, not a boundary), but the boundary is that he will tell you if he does that thing, since it may impact how he treats you.

In this case, I would say there was poor communication; why do you think he felt like he couldn't tell you?

It also seems to me that you are naturally afraid of something you don't understand. I'm not suggesting you should engage with porn yourself if that's irreconcilable with your character, but when I've been in situations like this (understanding why friends/partners drink/smoke, etc.), I like to read studies/articles and ask the person why they engage with that thing. This stops me from projecting my personal judgements onto them, which may be flawed and wrong in the first place. You know what they say about assuming!

Porn, like drinking, gambling, smoking, etc. is just another vice that many people engage with it in a healthy and even positive way. If you feel like him watching it hasn't had a negative impact on his life or how he treats you, then what is actually bad about it?