I will clarify some of my thoughts and add some more detail to the idea of teaching pornography to children. Modesty is all about respecting our bodies that Heavenly Father has blessed us with (this is unstated because that is behind the idea of modesty anyway).
Let's start with the nature/nurture debate. There is none. Scientists long ago agreed that it wasn't a debate at all. Nature and nurture both interact at various levels throughout the course of someone's life to shape who they become. For example, biologically, a child hits puberty and certain hormones change the child's body and brain functioning. This biological phenomena can take place at different times in different individuals because of their environment (diet, race, stress, etc.). Instead of boring you with minute details, suffice it to say that this is an interaction of an individual's biological makeup with their environment.
Let's relate this to our pornography discussion and modesty. Modesty is for both BOYS and GIRLS. There is no debate. Pornography and the way people dress is like the nature/nurture interaction. By nature, we are drawn to look at people's bodies. The way someone dresses can either encourage more looking, or encourage a different type of interaction (nurture). BOYS and GIRLS are BOTH responsible for their thoughts, actions, and dress. Let's state the obvious, boys are less likely to dress immodestly than girls. Part of the reason for this is because when you go to the store, how many parents have a hard time finding a modest swimsuit for a boy? Not very many. However, it is significantly harder to find modest swimwear and other clothing for girls (especially when you consider special events like dances). Therefore, because there is a higher chance for immodesty among young women, and pressure to do so in our society, it is imperative that parents take the time to explain modesty and encourage their young girls to be modest. This does not mean that boys are excluded from this conversation. They also need discussions on modesty.
Next, let's consider the biological/social (nature/nurture) interaction. Boys and girls are drawn to bodies. It is unfair to expect that only girls or only boys be responsible for someone else. However, you do need to consider the consequences of some of your actions on other people. If a girl dresses immodestly, is only the boy responsible to "keep his thoughts clean" by always looking away? That leads to less effective interactions because the young boy is concentrating on keeping the biological balanced with the social (this is the interaction). He is concentrating, not on the girl and person she is, but on trying to avoid something that is distracting him biologically. The social suffers, which is imperative for finding a worthy, eternal mate, or practicing to find one (Dating). If a boy is dressing immodestly, is only the girl responsible to "keep her thoughts clean" by always looking away? Of course not, and for the same reason! We do need to be responsible for our thoughts, but in boy-girl interactions we need to consider how our actions and decisions affect other people. Then, we make wise choices based on our considerations. Modesty is in place so that the biological interacts in a balanced way with the social. Again, check out "The Drug of the New Millenium" about how pornography affects brain functioning. There are also a few other wonderful books about pornography, but few that deal so thoroughly with the brain process.
Pornography is all about lusting and viewing someone that is not dressed or acting modestly. So, modesty is a tool parents have to teach their children about these wonderful, amazing creations that Heavenly Father has given them. It's the best tool we have to teach ALL of our children about respecting bodies, whether theirs, or someone else's. Then, it will be easier for them to avoid pornography as they clearly understand the principle of modesty.