Well after doing some reading the medical community does not know what causes it.
Could be:
-DNA
-You are born with it
-learn behavior.
-environment
-genetics
-people around you are into it.
-by looking at fetish web site.
Now how does your brain process soft porn,hard porn and fetish or does it process it the same.
I check that web site and there does not seem to be too much information on it.