personally i’m not a fan of watching porn nor do i like it when my partner does, but we have established these boundaries during the first few months of our relationship. so in my case, it’d be pretty upsetting if he were to hide it. however, if you are okay with both of you watching porn - that’s totally okay too. if you aren’t, and you have made this explicit, i would try to start an open and honest conversation as to why they felt the need to hide it.