Time for a basic science lesson. Soaps are a base which is the opposite of an acid. Basically how a soap works is by disolving the top layer of skin and taking everything else with it. That is one of the reason soap feels slick the disolved epidermis, and skin oils are slippery. The second reason that it is slick is because you use fat to make soap. See <I>Fight Club</I> that is basically how soap is made except soap companies use the waste fat and grease from fast food resturaunts and such.
Now using that information you shouldn't use soap to masturbate for two reasons. One if it gets in your urethra (pee hole)it will damage it. Second soap doesnt know to stop disolving your skin after you have washed off all the dead skin cells so it starts in on the living cells and can get quite painfull.
It probably makes a difference what brand of soap you use. What brands do you guys use? I never use soap to jack off, but I use Dove - 25% moisturizing lotion. I heard girls like the smell of Lever 2000 best, but it wasn't a real extensive poll. Sometimes I use that.
I tried to use soap to jack it off before..... I had some problems, the shaft on my penis have so called skin tear. I was told by a male nurse that if we use soap to jack it off, it somehow will cause the skin teat problem.......