Does masturbation cause the white matter in brain reduces?

Yes, in time... If masturbation becomes a compulsion of life, it overdrives the pleasure-seeking cerebral cortex in time leading to its suppression and shrinkage. More detail on its related subjects? Go to http://www.lifeissues.net/writers/hil/hil_01brain.html. Besides, treat compulsive masturbation as a drug addiction from overindulgence and obsession as elaborated in http://formefirst.com/eNewsletter05.html.
Masturbation. I am unaware of any studies that masturbation causes any degenerative changes on the brain.