Why did he have to sneak and lie about it? Because you didn't want to let him have it? Because you made it into an issue that you could fight over?
Because for someone with that kind-of addiction, it is partially the thrill of the secrecy- of the naughtiness. He wanted it kept secret. It was his little thrill- sneaking around getting porn and using it. No matter what was allowed in the open, he upped the ante in private.