It was a fictional assault on a show full of fictional assaults. The very first season (one of the early episodes) had a similar scene, involving a girl who was 11/12 in the books (but made a bit older for the show, but still implied to be underage). Talking about Daenerys Targaryen and Khal Drogo. I think that was worse as her character really had no idea what she was getting into, her brother was basically calling all the shots at that point and she was going along with it because he'd beat her (and worse) if she didn't play along. So she's playing house marrying this Mongolian dude, and it's all fun and games until he takes her out and ravages her, holding nothing back. Never mind that he turned out to be a good dude, more or less, that was still pretty brutal.
But honestly, you kinda know what you're getting with that show. You don't watch Walking Dead if blood and gore makes you squeamish. Neither of them are cultural classics that really need to be seen by anybody.