Control. Most religions establish a set of behavioral rules that conform with their idea of what their particular deity and elders feels is correct behavior. The constraints on masturbation fall in this category and were established usually long before disease became a connected issue.
Wrong Forum. Medically, there is nothing wrong with masturbation. It's completely normal. But your question is above my pay grade. Best source for further advice on this topic may be monty python's "meaning of life.".
Is that so? Speaking as a protestant christian, i'd like to reassure you that that vast majority of us, including our leaders, have no problem with you enjoying yourself in private. People who warn you of "spiritual dangers" here are perhaps simply trying to make you afraid to sell a dubious product. Talk to someone who's a member of your faith community and has some common sense ; decency.