There is no "reason" behind it, its become a parody of itself however and I doubt many (myself included) would be caught dead limp wristed.
However there are effeminite gay men out there, to which the limp wrist is associated and I am sure some have a literal limp wrist but I doubt its something that is thought about and done on purpose.
I am also sure there are people who do it on purpose, used in part parhaps as a way of letting people know they are gay, but a limp wrist action alone is hardly an indicator of someones sexuality.