QuestionIf someone with only one testicle lost his semen by hand every once in a while, does it cause any harm?
The male body is designed to have ejaculations. The semen you ejaculate is mostly produced by two glands inside your groin, near your bladder, which are called the seminal vesicles. Your testicles only contribute a very small amount -- the sperm -- which in volume is less than a drop. So the fact that you have only one testicle doesn't make a difference.
Losing semen by hand is called "masturbation." The physical act of masturbating doesn't cause any harm, but there are concerns about what young men think about in order to get themselves aroused enough to ejaculate semen. See: "Is masturbation unacceptable?", "I'm still confused about masturbation. Is it right or wrong?" for more details.