You raise an interesting question. I think that the progression starts with a simple awareness of a concept of a circumstance. By dwelling on the concept or seeing the possibilities in a circumstance a mild desire can develop. But the progression accelerates rapidly once the thought is used to fuel masturbation.
In my own case I have two fantasies that I have thought about during masturbation and both concepts went from a mild stimulation to a burning desire once I let it lead to orgasm. It is the link in the brain between the concept and the release of endorphins and other pleasure chemicals that causes the desire to accelerate.
I have not acted on either. One because of circumstances and the other because I know it would be unwise.
