To go further, almost everyone agrees that a cartoon of a man and woman having sex in the missionary position for the purpose of procreation is not necessarily pornographic; this is a standard sort of picture in age appropriate sex education materials.
Granted, these images concern sexual reproduction. However this only makes me question why people deem that image appropriate, but images of other positions or acts not worthy of learning about in other contexts.
The fact is that we do learn about sex at school for other reasons. "Safe sex", consent, and in many cases contraception are all taught.
So hypothetically, if we're fine with images to educate about reproduction, and images to educate about consent or safe sex for nonreproductive purposes, what makes the images in the book so different as to deserve to be forbidden knowledge?