Masturbation doesn't cause a 'dent' in your penis. If it's a curve in your penis, it's beacuse of genetics. If there is a dent, it's because your penis has undergone some sort of injury, and developed a dent in it as it healed.

Curves and dents can work in your favour, because a curved or dented penis can stimulate areas in a woman's vagina that a straight penis couldn't otherwise stimulate. Use it to your advantage! (As long as it's not causing you pain or discomfort).