It's interesting that some people have pointed to the Bible account of Onan as a statement against masturbation, whereas it obviously refers to his practicing "withdrawal" to avoid impregnating his wife -- but the Witnesses, to my knowledge, have never used that account as a basis for their anti-masturbation stance. This leaves them with nothing concrete in the Bible to point to, so they can only make assertions that masturbation can be a gateway to more sinful thoughts/actions.
To be fair, most people today probably use porn as a masturbatory aid, and this can lead to more sexual thoughts because you are taking in lots of visual stimulation, and seeing things that you might otherwise not even have known existed! And the pleasure of viewing porn, like any pleasure, can be addictive. Whether masturbating leads to viewing porn, I don't know, but I think that for a young Witness, it's within the realm of possibility. He reasons with himself, "Well, I'm already sinning by touching it regularly, I might as well look at this arousing material while I'm sinning since I can't get any worse."
But if you look at the simple reality that it's nearly impossible not to have these urges, and that sooner or later one will give in to them... Witness youths are being set up for a world of hurt. I suppose the youths, like Heartofaboy's nephew, who don't take this teaching seriously, are the lucky ones. But as someone who was trying to earnestly be a good Witness and follow everything he was taught, this issue nearly made me kill myself. I can't forgive a two-faced teaching like this being imposed on naive kids like myself who didn't know that the people who wrote it probably did the 'dirty deed' too! I honestly thought that I was the only person I knew with this problem. I'm actually somewhat surprised that I'm even alive to talk about this. Thank God I moved past the guilt a long time ago.
P.S.: Like smiddy, I am genuinely curious to hear from some women on this (it doesn't need to be personal information, you can tell us what your female friends have told you). Was this a big problem for young sisters growing up? It's always seemed to me that it was more of a problem for guys... probably because the erections we experience lead to curiosity, and one thing leads to another.