I agree that the dad should have looked at the selections when his son brought them home.
But, I think the dad had a point about the library needing to enforce the adult section.
There are ratings on the books for a reason, just like movies and videos. Theatres aren't supposed to let underage kids into "R" rated movies, so the library shoudln't let kids into the adult section.
If they are going to have an adult section in the library, then there should be someone there to monitor it. During the summer, especially, there are kids' programs at libraries where the parents drop the child off for a couple of hours then come back and get them. Even if this child didn't bring the books home, he could obvisouly view them in the library setting.