Incest, rape, murder, sex, child/spousal abuse, racism, sexism, poverty, religion, politics, war.
All of these things happen in real life, why then should we be upset when they occur in literature?
Good literature is not necessarily literature that makes you feel all warm and squishy inside. There are characters that are terrible and who commit heinous acts just as in real life. Should art not imitate life?
Stories like Huck Finn, The Scarlet Letter, To Kill a Mockingbird were all once [sometimes still] challenged by society. Often fictional settings can teach a reader a moral without being confrontational. Why take a valuable teaching tool away from the public in order to satisfy a few?
There is a slippery slope between declaring a topic or book is too controversial or offensive and outright censorship.