maybe Webb enjoyed writing his books, but I'll tell you the main reason this stuff gets put in books: To sell books. I find that almost every book I read, even serious conservative topics, have liberal or sexual stuff thrown in. To make it acceptable to the NYT and to make it less boring to various shades of libertines. Webb wanted to sell books, some of it may not play well in a Senate race.
Sexual scenes are one thing, sex with children is VERY DIFFERENT.