The thing of it is, there is no rule against "sexist", while I do have a rule about pornography. However, pornography is subjective in nature. I agree, posting the picture and adding the teener comment seems to objectify a woman (specifically the one pictured, anonymously with no face). However, IMO, it doesn't constitute porn as if you saw that in a movie, it would still not make the movie a "porn" flick. Let's say they were talking about the book on a talk show or some other show on TV. While it would rate a TV14 rating, it certainly wouldn't be disallowed on TV. It doesn't show any of the important bits that would make it something that would be "porn", soft or hard.
Sexism does not equal porn. They are not synonymous terms. And like the entire US, this particular site has no guarantee that you will never be offended by what is posted here. In fact, it is almost certain that you will be offended at times from what others say. One should not assume that the staff is here to protect them from being offended.
"Porn", being relative in nature (and unless it is about kids) will be treated subjectively, as there is no other way to do it.
Ask yourself, if they were to grab this book in a movie and start talking about it exactly as 3D did here, would it make the movie even NC-17? Would it even make it stronger than PG-13?