There is, as we speak, a kerfuffle over the book "Genderqueer", and its presence in grade-school/high-school libraries, and whether it should be legal to sell to children.
The book contains one or two illustrations of fairly clear sex, including a perfectly clear illustration of a blow-job.
The book (as I understand it, I won't have a chance to actually read it until tomorrow), while direct in its storytelling and visuals, is not pornographic (unless you believe any depiction of sex is pornographic). It's a coming of age story of an LGBT teen.... and I think we need more of those, and for those books to be as normal as every other coming-of-age story.
My priors: were it up to me, I would let any child who can sort out what the words mean read any book they want.... and that includes books that contain the occasional "mature" illustration. I realize I'm a good deal more permissive about reading than quite a few other Americans are, but I think Americans practically make it a point to screw up their kids on the topic of sex, especially when it comes to any of the infinite variation that isn't covered by "two straight people groping each other with the lights off".
I ask you to keep in mind that we live in the 21st century.... if you believe that keeping a book like "Genderqueer" out of your child's hands is going to prevent them from seeing images of oral sex, you're either naive or Old Order Amish. It is better for kids to stumble across sex contextualized as a human experience than it is for them to discover it via detached-of-human-interaction internet porn.
I recognize my standards for what kids may read are not the same as other people's standards (because, ahem, too many of you have forgotten what it's like to be an inquisitive kid), so:
What age (ish) is it acceptable for kids to have access to books with realistic and reasonably graphic written depictions of coming-of-age sex and the occasional graphic (though not particularly titillating) illustration?
(And my bias here is that I think kids are more capable of understanding than most adults give them credit for. I was in middle-school when I read "Flowers in the Attic")
Again, I urge you to consider that these pages are in the context of an entire book about coming-of-age. Do not reduce this book to a single illustration.