Since word "masturbation" is the scientific and clinical term for self-stimulation, I don't see how it could be considered derogatory. When something is derogatory it is (according to Dictionary.com) "tending to lessen the merit or reputation of a person or thing; disparaging; depreciatory." I'm uncertain how calling it masturbation lessens the merit or reputation of masturbation. Actually, I'm fairly certain that she misapplied the word derogatory.