Guys (excluding Lei...I'm sorry to hear about your death by selfpleasure, you will be missed), that was a joke. In all seriousness, I masturbate probably once a day on average, and the same stands true, no noticeable effect on anything, including sensitivity.
On a more serious note:
I remember reading a study on this a couple weeks ago.
It was led by Graham giles, of the Cancer Council of Australia. Apparently, chemicals that cause cancer can accumulate in the prostate if a guy doesn't "get off" regularly. Their idea is that masturbation is better than regular sex. Sex doesn't have the same protective benefit due to the risk of STDs and diseases.
"It's a prostatic stagnation hypothesis. The more you flush the ducts out, the less there is to hang around and damage the cells that line them."
^ That's quoted from somewhere. I remember the quote, but not the source. As far as I recall, none of it is unquestionable scientific fact, but I can't see how you could go wrong by masturbating once in a while as a preventative measure.
Unless you subscribe to Immanuel Kant's
a priori concept that states masturbation is a disservice to your true self in that it suppresses your humanity to satiate a base animalistic desire.