The author didn't moralise about the situation, leaving that concern to the character Humbert, and that's actually the entire story of the book, how Humbert handles moral questions. Humbert's endless twistings and turnings to justify/excuse/pardon/accept blame for his behaviour are the real subject under scrutiny. I felt like the author made this obvious enough with the 'foreword' by a fictional editor who calls Humbert "a shining example of moral leprosy" and "ponderously capricious", I think any further underlining of the point would take away from the strength of the book.