Is it safe for a man to use a vibrator on his balls?
Yes. Occasional mild vibration of the testes should not damage the testes or
affect their function.
Long Answer: An Austrian study found links between mountain biking in men and decreased fertility and testicular abnormalities. The connection was the strong and repeated shocks to the scrotum and perineum (the area between the balls and ass) that the men were subjected to while biking. However, compared to the bone-shaking vibrations of bouncing down a rocky trail or jumping stumps with a bike post planted in your crotch, the minor tremors supplied by a typical vibrator are quite gentle and non-traumatic. Overuse of a vibrator on the testicles is not advisable, but the occasional buzz on the balls shouldn't do any harm.