I don't believe that modesty is natural law "written on everyone's heart".
?? You're serious? Of course it is.
What happens to those who act immodestly and claim they "don't know", is that they commit other sins regularly (i.e. lying, uncharity, laziness, lack of prayer), so their conscience is dulled to all manner of things, including modesty. A person with a well formed conscience is acutely aware of modesty because they are in-tune with religion. A person who is not living their Faith, or doing so lukewarmly, will have an ill-formed conscience, which is ultimately, ignorance of sin.
Just like the native indian who doesn't know about Catholicism (i.e. because he doesn't practice the natural law) and does not make it to heaven...this is a punishment for sin.
So a Catholic who doesn't have a well-formed conscience (i.e. because he doesn't pray or lukewarmly practices his Faith) and does not recognize sins against the natural law...this is a punishment for sin/lukewarmness.