Masturbation isn't directly mentioned in the bible but neither is smoking.
Many of the reasons given for disfellowshipping smokers seem to also apply to masturbation: a form of uncleanness, a defilement of the flesh, letting an addictive thing become king over us, a form of covetousness.
Why are occasional, habitual smokers disfellowshipped and shunned, yet chronic masturbators, of which the organisation is of course replete, do not even get a slap on the wrist (excuse the pun)?
yadda