Several reasons:
Law of averages... The consensus of those doing a statistical analysis of who masturbates is that around 90% of the total male population do it all the time... (the other 10% are lying...) and 65% of the total female population masturbate from time to time.
Watching women masturbate is HOT! Watching guys masturbate... who cares...
How many sex websites of the internet showing women masturbating in HD quality pics and video... compared to men... again- who cares. :???:
Women are far more inventive in the way they masturbate... e.g. pulsating showerhead! :shock:
Therefore, it counts for women... not for men.
I don't make the rules... it just is.