If asked to name a sexual condition they considered "unusual," many people would probably point to fetishes -- known as paraphilias in the psychiatric community. As defined by the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), paraphilias are mental disorders "characterized by sexual fantasies, urges, or behaviors involving non-human objects, suffering or humiliation, or non-consenting persons." Generally, paraphilias are extreme and fall well outside of the norm, and paraphiliacs typically can't find sexual gratification without a particular object or act involved. Particularly when they focus on something dangerous or illegal, these conditions can be severely damaging.
The controversy is whether some paraphilias should really be classified as mental disorders. For example, some argue that a paraphilia involving consenting adults that doesn't cause distress to the paraphiliac or other people isn't really a mental disorder. Transvestic fetishism, in which a man is sexually aroused by wearing women's clothing, is often cited as an example.
Researchers aren't sure what causes paraphilias; they may stem from behaviors or associations made early in life. Many people with paraphilias don't get any kind of help with their condition until it's discovered by other people. Along with behavioral and cognitive therapy, counseling has helped some people stop acting on their paraphilias, and so have medications such as Ritalin. Generally, though, paraphilias are very difficult to treat.