Young Christian idealists often have had some brushes with debauchery, like St. Augustine, and they are partly seeking a way to control their harmfully licentious impulses. Control of masturbation seems to come up as an issue, since it seems to relate to control of promiscuous or degrading sexuality. Its simplicity and immediacy makes some people feel that it may be at the very root of self-control - control this and you will learn to control all. Rather like starting your two-wheeled biking career on the little bike with the two training wheels.

To me, this is all illusion. In fact, it's worse than an illusion, because it abstracts self-control out of any context of ruling one's life by distinguishing loving from meaningfully selfish acts. (Just because masturbation can be done alone doesn't make it selfish - that's merely coincidence of English. We could say "avaricious" instead and get a better grip on the distinction.)

However, since people really are grasping in the dark to get a grip on self-control, the symbol of masturbation will come up over and over again spontaneously. So the function is very hard to divest of its inappropriate symbolic encrustations. (With no pun on crustiness intended).