I've been watching my dad play Resident Evil for as long as I can remember. My parents are split, and my mom never allowed M games in the house. In all honesty, she didn't like video games in the house in general...No fighting games, ever. Definitely not anything like Mortal Kombat, but Tekken was banned too...Even Super Smash Brothers was banned from our house. But my dad let me play whatever I want, and watch him play all the M games all the time.
Now my little brother on my dad's side is 6, and he plays M games all the time. He's played so much Resident Evil, House of the Dead, God of War, it's insane. But he's fine. Anything sexual in any of the games, he doesn't understand anyway, and my dad did the same with him as he did with me. Told us it's just a game, there's nothing to be afraid of. It's not like my little brother is acting violent because of it. It's not giving him negative thoughts, not making him anymore bloodthirsty that we are as humans, anyway. He's the sweetest little boy, and the oddest thing he's done since is act out a few of the games. He jumped into a pool like Kretos tries to jump off the cliff and the end of GoW, he's pretended to shoot zombies a couple times with nerf guns, but how is that different from any other stereotypical little boy?
He of course, still plays Mario and Zelda and his Mickey Mouse games, but what is he hurting by playing the games?
My dad and step-mom aren't stupid or irresponsible for letting him play, they're a gaming family. They could be doing a lot worse by him than they are by letting him play a few M rated games...