The threat of having the police called and getting added to sex offender lists is more than enough to prevent all but the mentally ill from masturbating in public. People can already access pornography at just about any coffee shop (or for that matter, wherever there is cellular phone access...), but somehow we don't see outbreaks of uncontrolled public wanking. You seem to be operating under the assumption that systems like this are already widely deployed. They are not.

And as for the mentally ill? Well if walking down the street of your average sized city is anything to go by, lack of pornographic material isn't much of an inhibitor.