When I was younger, Duke Nukem 3D had a "parental lock" option with a passcode. Naturally, as any inquisitive child would do, I backed up my save files, uninstalled the game, re-installed it, and set up a new jibberish password that way my parents would assume they forgot the code or there was an error with the game.
With or without the adult content, Duke was one of my most cherished shooters. Although it is decidedly dated by today's standards, its massive 10-weapon loadout featured a ton of diversity, and the environments contained some of the best hidden areas in the genre, even today.
It's a bit of a relic, but Duke Nukem 3D: Megaton Edition is an endeavor worth pursing if you have any interest in first-person shooters, and can deal without a handful of modern conveniences.