<i>"Moderate masturbation has no negative side-effects, unless one counts an overabundance of sock-related laundry. Positive benefits include stress relief and exercise for the prostate. (Women's masturbation doesn't even come with the necessity for doing an extra load - heh - of laundry.)
Self-harm, on the other hand, introduces potentially dangerous bacteria to the bloodstream no matter how rarely the cutting occurs. A cutter could nick an artery. The practice also leaves scars.
Death by simple masturbation is pretty much unheard of. Death by cutting, on the other hand..."</i>
We weren't talking about female masturbation so I'll disregard that comment.
A person cutting themselves would not do fatal damage unless they tried really hard... In fact it is so hard to kill yourself with a knife that many suicidal women in Iraq, Afghanistan, Pak and India (those are lands a certain religion has really fucked up) prefer to set themselves on fire.
I'm speaking as someone who has personally observed this "cutting" phenomena. It hardly ever leads to serious harm. Your point about bacteria is equally facetious (unless perhaps they use a rusty or dirty knife, which they won't unless they're genuinely trying to harm themselves as I said) I mean, a man masturbating could rub himself raw and thereby open a far greater area of skin to infection than a little cut.
And yes I know most people who cut themselves use disinfectant and bandages - it's not about causing genuine damage.
So if you think masturbation is OK you must feel the same way about cutting.
Cheers