Postal 2 has an M, but if any game deserves an AO rating it's this one. It's an open world game where you have a a specific key bound to the 'Unzip Fly' command. Playing this game is descending into the misanthropic nightmares of Jack Thompson, with a far more visceral experience than third-person shooters can offer.
But good lord, is it fun. The developers have thrown a huge amount of stuff in here, and the world behaves more like the real world than most gamers are probably comfortable with. There are true decisions; sure, you could become a serial arsonist, or sodomize a cat with a shotgun, or you can complete the entire game without wielding a single weapon. The Postal Dude is only as violent as you are. With the Unreal 2 and Havok physics engines, modified with full fluid dynamics, it's somewhat dated but more than capable of delivering the goods.
Finally, I want to share an incident that defines the Postal 2 experience for me. In a desert canyon, I had beheaded a drifter with a solid blow of my shovel, and was kicking the severed head down the road when a stray dog ran up to it, picked it up, ran over to me, dropped it at my feet, and looked up at me expectantly. I spent the next ten minutes playing fetch with a stray dog and a HUMAN HEAD, tears of laughter rolling down my cheeks. This was not a scripted event, this is just one example of the kinds of things that happen spontaneously in this game. Try it, but for God's sake keep it away from the kids.