Why the Shame for Masturbation?

So it seems that there is a definite stigma associated with the act of masturbation in our society. It is a dirty thing, not to be spoken of in proper society, something to remain behind closed doors, forever a closet phenomenon. I can't say that I would necessarily want to change the convention surrounding masturbation, but it would be interesting to see what life would be like if it were an open issue.

Imagine the awkward talk with a son. On a Saturday afternoon, there is little to do in or outside the house because it is raining. A fire has continued steadily in the living room, giving the house a gentle warmth to contrast with the snow outside. A commercial pops up on the television and you decide to start unloading the dryer of its whites. After all the whites are sorted and delivered, there remain a number of socks to go to your son. You walk up the stairs quietly and open the door to catch him stroking as a dog would the back of his head for a particularly bad itch.

What happens? What is all of this? Something to be ignored and put aside? How does one go about reconciling the mutual embarrassment? First of all, masturbation is said to be wrong. Here's the big question: why? Who is it exactly that determines masturbation to be a sin?

There are a few mentions in the Old Testament of men emitting semen, including Leviticus 15:16 which says "When a man has an emission of semen, he must bathe his whole body with water, and he will be unclean till evening" (NIV). Leviticus 15:18 goes on to say "When a man lies with a woman and there is an emission of semen, both must bathe with water, and they will be unclean till evening."