When your genitals are mutilated (aka. circumcision), you can not receive full pleasure from sexual activities like masturbating. You ALWAYS have pent up sexual frustration. Think of it as an itch that you can never fully scratch. This is why the brain tends to go to the extremes, via fetishes, to find an artificial satisfaction.
I don't know about ''cock mutilation fetish'' but it definitely contributes to femdom.