I personally don't believe it should. At all.
The only thing you see is the book cover. This is the same thing a child could see in a bookstore or someone's bookshelf. Do people put locks on their book shelves? Do we have to start carding kids before they walk into a book store?
If parents want to "protect" their childeren from world around them, they can use parental controls. I know this may sound harsh, but this is not Amazon's job to do.
Besides, as Trauma Doll said, you only get erotica in your recommendations if you actually purchased erotica. So chances are erotica is already in your library and thus easily accessibe to your child. Just sayin'...