His aunt watched on

Dave had barely recovered from one orgasm before his hands resumed caressing his body. One hand slid over his aunt's fat butt, squeezing the fleshy cheek before circling her tight puckered hole.
Dave threw his head back and moaned, feeling his hair falling down his neck and the breasts shifting across his chest as he wiggled and flexed his long legs.
Dave's aunt had been at the house when the Stranger burst in on them. The Stranger had used his power to make Dave tell his deepest secret, and Dave had found his mouth opening to blurt out "I want my aunt's body." Dave's aunt was hot, and he'd secretly imagined fucking her, slipping into her pussy from behind and watching her tits jiggle as he pounded her.
But the Stranger smiled and clicked his fingers and suddenly Dave found himself in his aunt's body, his desire granted in the most literal way possible. Dave and his aunt, now in each other's bodies, froze in shock and disbelief.
"Go ahead," the Stranger said, "Enjoy yourself."
And with another click of his fingers Dave found his hands suddenly roaming around his new body, squeezing his tits and stroking his supple skin. He couldn't stop feeling himself up, and his aunt was forced to watch from his body as he slid off his pants and began fingering himself to orgasm after orgasm. The pleasure was immense, and he groped and squeezed and fucked himself as his aunt watched on, both of them trapped by the Stranger's power.

A freak accident swaps a young man into the body of his best friend’s girlfriend where he discovers his masochistic side in Better Than Ever, available on Smashwords or Amazon. Preview here.