YAMCS (Yet Another Mind-Control Story). I see these options:
* For fun and strokes, he just goes on fucking and fucking, no limit.
* For focused fun, he's only turned on by certain qualities: midgets, BBWs, amputees, his family, hairballs, psychos, cat- or bag-ladies, etc.
* For drama, he loses something of himself (lifespan, vigor, IQ, length, sensation) each time he exercises The Power.
* For perplexity, The Power switches on and off at random.
BTW no magic is required. He has a genetic mutation which, when triggered, causes production of Irresistible Charisma and Horniness pheromones. I worked with such in Randy Randy (The Pornomancer)
. How does unlimited sexual power affect a person?