Most sex educators would agree that knowing our own sexual desires is an important part of sexual health. Simply having a desire or a fantasy is not the same thing as wanting to act that fantasy out in real life. For many people it can be perfectly healthy and safe to explore even extreme desires and fantasies through masturbation. If you are someone who feels distressed by your sexual desires, or out of control, then talking to a mental health professional is probably a better way to go. But if you just aren't sure where your limits are, using fantasy and masturbation can be a great way to map out those lines and understand what it is that turns you on.