Take "Young Adult" fictions, for example.
You hear the Twilight was a good series of books. Your daughter enjoyed them. So, as a present, you go to the store and get her some other Young Adult vampire fiction, only, unbeknownst to you, this Young Adult book is filled with sex scenes. You had no way of knowing, unless you'd read it.
I think, in many circumstances, such as 'inapproriate' or 'abnormal' content, it should be stated. Perhaps on the back. "This book contains graphic depictions of sexual encounters and abusive or harsh language."
Now, should this warning be on an adult Harlequinn book? Of course not, it's expected. Should it be on an Anne Rice novel? Only if there was a LACK of sex.
