Don't let this happen to you. Become informed about chemical and surgical options. One of the things I enjoy about working with men goes like this. Men have very independent personality traits and characteristics. Men also lead very rich fantasy lives.
Some men however play act castration fantasies during masturbation, for example.
This can be dangerous. Participants may frequently act on self-castration fantasies by using rubber bands, knives and other devices. My suggestion for this behavior is play safe. Make a promise to yourself to avoid directly cutting on skin. If you have self-mutilation fantasies that lead to cutting, its time to seek counseling or sex therapy.