Weakness in penis due to over masturbation is happening because whenever you ejaculate you use certain muscles and certain nerves. Using them over and over again without leaving them time to recover will only damage them and make them incapable of serving their purpose. Over masturbation can have the following bad effects: 1. A mental imbalance characterized by an addiction towards the habit. 2. Delayed arousal or absence of arousal in case normal man-woman copulation. 3. Difficulty attaining orgasm in normal heterosexual relationship. 4. Inflammation or abrasion of the male sex organ