Originally Posted by moonlite
Why do the most spiritual disciplines forbid masturbation?
Because the sexual drive is so strong it creates an excellent opportunity to train your will.
First of all it would be nice to define what do you mean by "most spiritual disciplines". You didn't say religions, which would limit us to just 4000 ones to check out. I'm not sure that most spiritual disciplines in fact forbid masturbation.
But those who do, do it for various reasons.
One is to put everyone in a position of a sinner. By the way even in Christianity, masturbation is not a sin and is frowned upon because of Onan contradicting the custom of having sex with his dead brother's wife.
Once you have everybody at a sinner's position it creates great amount of control.
In Islam masturbation is piled together with any extramarital sexual activity.
This is also a way to control the followers.
In Hinduism nothing is said against or for masturbation.
Neither in Buddhism
In Taoism masturbation is encouraged as a practice that leads to the control of ejaculation and sexual energy.
Some schools might prohibit masturbation for the reasons of building character, but I'm not sure about that.