Masturbation.

To be perfectly honest, I really can't see why masturbation is regarded as bad. It's a natural activity which people, most of the time teenage boys, take up to relax, relieve sexual tension and sometimes just to fill up time. It's a very private activity, most of the time, it doesn't hurt anyone, so I don't understand why some people see it as a problem.
No matter what your gender/age/race, people masturbate. Some people do it everyday and more. It's become such a notable pastime that its earned a wide variety of nicknames common with most teenage social groups. From 'Jacking one out' , to more imaginative phrase's such as, 'Choking the Chicken' or 'Bashing the Bishop.'
There are even website dedicated to this activity. http://www.masterbationlounge.com/
On the subject of religion, this is the main reason why its frowned up. The Catholic church believe that masturbating is as bad as using a condom, they believe that your wasting a life.
The best example of this view is in the satirical Monty Python sketch from 'The Meaning Of Life'.