Warnings: Fisting-related kink.
Notes: I got sick of seeing people misuse the term "Fisting" so I decided to write a fic to show what fisting really is...
Billie struggled against the leather straps binding him on his back to the metal bars on the headboard. Mike was up to something, and not knowing what it was only served to arouse him even more than he already was. His naked body was covered in a thin layer of sweat, glistening off every curve of his body. He could hear Mike rummaging things around in the bathroom. Occasionally something would fall, Mike would cuss, and Billie could briefly see his naked ass poke out the door as he bent to pick it up. It would be amusing if it weren't for the fact he was so horny he could hardly see at all. After a moment of silence, Mike returned to the room with a bottle of lube and his hands coated with latex gloves. Billie's eyes widened as he realized what Mike was about to do...
Mike knelt down on the bed next to Billie and started turning him over. Billie didn't resist and, in fact, almost threw himself onto his stomach in anticipation. His wrists were now crossed one over the other but it was more uncomfortable than anything. He tucked his knees up under himself into his stomach and waited impatiently as Mike moved to kneel between Billie's feet, right at his ass. Mike lubed up his fingers and inserted one, moving it around, loosening Billie for what was to come. Slowly, he inserted two more, twisting his fingers and rubbing him from the inside. Billie's muscles contracted around Mike's fingers, but Billie knew if he didn't relax Mike wouldn't continue because it would hurt Billie too much. Three fingers weren't a damn thing compared to what was coming and three fingers fit almost loosely inside him. Mike worked a fourth finger inside and then pulled out. Billie whimpered and buried his face into the pillow under his head.
Mike popped open the lube and coated his hand liberally until he could see the layer, thick and gleaming, over every surface of his hand. Slowly he worked his four fingers back in, rubbing Billie's prostate, and then pushed further until his thumb was inside as well. Billie's moans were muffled by the pillow but he then moved his head to the side to breathe, chest heaving and back arching. Mike waited a moment for Billie to relax before he pushed further until his entire hand was sheathed in Billie's ass. Billie let out a long, loud, animalistic moan and tried to push back on Mike's hand. Mike's free hand pushed against Billie's ass to make sure he didn't push himself back too far; the latex glove didn't cover that much of his wrist. Mike slowly pulled his hand partially out before thrusting it back in a little quicker but not at all fast. Completely inside Billie, Mike slowly curled his fingers into a fist. He pulled his fist out about halfway before pushing it slowly back inside making sure he was leaning into Billie's prostate. Billie was quivering, moaning, almost crying out in pleasure and pain. His hands clenched and unclenched above his head and his wrists strained against the leather straps as Mike continued his slow but steady pace.
It only took a few more "thrusts" before Billie screamed out Mike's name, his muscles clenching painfully tight against Mike's fist. Mike pulled his hand out and, with his free hand, pulled Billie back to his original position laying on his back on the bed.
Billie no longer struggled against the leather straps binding him on his back to the metal bars on the headboard. He was, though, covered in another thin layer of sweat, glistening off of his ribcage as it heaved up and down with heavy breaths.
In case you couldn't tell, "fisting" is a kink in which the entire hand, fisted, is used for penetration. Please do not say that they are fisting their penises because, honestly, that is quite impossible.
More info? Check out Wikipedia and Minotaur's Sex Tips for Slash Writers (NC-17 for pictures (yes, pictures lol) and graphic descriptions).
I highly recommend the last website for slash information. It is written by a gay guy specifically for straight slash authors so it's an absolutely awesome resource.