ok a direct quote from the page for fuck is "Other uses are dysphemistic: The sexual connotation, usually connected to masturbation (in the case of "go fuck yourself" or "go fuck your ass") is invoked to incite additional disgust, or express anger or outrage. For example, "Fuck that!", "Fuck no!", or "Fuck you!"." 
thats a bit worse than saying "masturbation is like taking a good dump" mate come on just put it back up and quit being a douche...