I'm not prudish, but we are talking about children, and younger teens accessing things we think may be inappropriate for them. To that end, I think content sellers should have some responsibility considering the anonymous nature of an electronic sale. If nothing else, the simplest way to keep teens from purchasing/accessing content would be to simply offer a password protected option that would allow a parent to check the current reading list. This puts the emphasis back on the parent to do the parenting. Informing your child that access to their reading list exists lets them decide what to risk and leaves it up to you whether you need to check it or not. It might even allow the parent who gives a darn to get their kids to discuss with them concerning things they might like to read. I suggest this because trying to determine a content rating system for literature is as laughable as setting up movie ratings, etc. You know, once upon a time children and even teenagers were accompanied to the movies by their parents, many even watched television with their kids -imagine that!
Now, on the other side of the coin, being over protective is likely to result in a backlash, secretive behavior, and over indulgence the first chance that comes along. Whether we wish to face it or not, once puberty hits there is no putting the genie back in the bottle. The parents who make it known to their children that they were once young(gasp) and try to relate probably end up with the better balanced kids who knowing pros and cons are prepared to make judgements rather than just act on impulses. Sheltering your children too much will do them no good whatsoever, you'll merely retard their growth and then thrust them into an unfamiliar world when they leave the nest, and preparing them to leave the nest one day is a large part of the job as a parent, at least as important as protecting them until they do.
A final thought, you may think this strange, but one of the most humanizing things my father ever did was telling a dirty joke with me included in the "audience." The poor man tried so hard to be the proper upright father, and he was -my parents were happily married over 50 years and to my knowledge never spent a night apart except while hospitalized, but he became "human" to me that day. I guess what I'm saying is don't be afraid to let your teens see that you're as human as they are -well maybe not quite as human...
A little honesty will go a lot farther than trying to perpetuate the perfect parent myth. By the way, let 'em make their own mistakes and learn from them while the mistakes are still little ones. You made your own didn't you? Do you honestly recall very often learning from anything other than mistakes? Think about it.