Are you experiencing dissociation when having a perverse sex fantasy?

Dissociation? I am not completely sure what you mean by a "perverse sex fantasy" (eg., bizarre sex, homosexual sex, bondage?) we all have some fantasies that we're not completely comfortable with. It would be dissociation if you're with a partner and become completely unaware of him/her due to your fantasy, or when, if alone, you behave in a way that is totally incompatible with your normal personality.
No, just fantasizing. Dissociation requires amnesia and an action on the part of another personality state, which you may or may not know about.