Well (un)fortunately that's the day and age we currently occupy. Social norms will not allow for such a relationship without certain rules or stigmas being attached. While sexuality can be quite complex, for me it's quite elementary. Adults are suppose to be the overseers of right and wrong. Children, Animals, and the Environment all look to adults to protect them from harm. Just because a young person has the capacity for lust does not mean it should be met by it's target.
The movie American Beauty featured a middle aged man who lusted after his teenage daughter's flirtatious best friend. He struggled with the moral dilemma of giving in to his fantasies. A good movie to revisit