I'm not judging the use of sex and violence. It's that Game of Thrones (and obviously, A Song of Ice and Fire) actually have reason to put the sex and violence in there, the others don't.
Really? I mean sex is only there to distract you from the boring subplots while violence displays how cruel the world of a song of ice and fire. Lord of the rings has its own reasons to display sex and violence such as men and dwarves being lusty and mutilations caused to both good and evil. Although Christopher Tolkien would probably view an 18+ lotr spinoff as a dump on his fathers grave.