Lets have a look at this one shall we, havent read the thread so might be repeating someone else.
Im thinking the guy who came up with the term wanted to make sure it reflected the profession in which is was to be used
Adult= Grown up, adult, independant
Webmaster= Master of what he does on the web (i suppose, right?)
Im thinking we should have our own for porn
Maybe: Cant webmaster
or: Hiding webmaster
or: Teen webmaster