This is a myth. When a man’s body needs to release seminal fluid, this will come in a natural manner during nocturnal emissions (wet dreams). There is not a constant buildup of pressure that will harm an individual unless he masturbates.
The next time you hear a claim like this, ask the person to show you the scientific evidence to support what he is saying. Odds are, he won’t have anything convincing to offer you. In the scientific realm, it’s not enough to say “I saw a study once that said so and so.” Researchers not only need to perform rigorous scientific experiments, but they also should have their work reviewed by peers and published in a reputable science journal. The findings should also be repeatable, and confirmed by other similar studies. Unless these things are present, don’t give much weight to what others tell you they heard.
If anything, the scientific evidence seems to show that masturbation is detrimental for both men and women. Click here for details on that.