Anything done to excess can cause problems. The penis has a limit to the number of times a day it can acheive an erection, and if the demands made upon it are more than reasonable, it will develop erectile dysfunction. Some people have damaged the penis with excessive and vigorous masturbation. If you regard your penis as your friend, to be worked with and cared about, not your slave or your enemy to be beaten into what you want, then the chances of erectile dysfunction or any other sort of problem will be less.