There's a story I've written with the following:

Sam went to the massage table as Hannah cried and he grabbed the Vaseline. He brought it back and knelt down behind her. “You’re lucky we’ve been so generous with this stuff,” he said, as he scooped out a fingerful of the petroleum jelly and applied it to her anus. “Imagine how much more damaged you’d be here if we hadn’t.”

Hannah tried to turn her head. “Oh, please, no. Not in there again, please.”

Sam pushed his finger inside her rectum.

“Aaaah god no! Pleeeeese,” Hannah cried. She suddenly turned to Sam with a face full of tears and stared into his eyes. “What the fuck is wrong with you?”

 Sam stood up behind her then and stroked his cock with a Vaseline-covered hand. “What’s wrong with me, Hannah? Mm, let me think. Uh, do you have a spare couple of weeks?”

He put one hand on her shoulder and with the other guided his stiff cock towards his goal – the glistening red rosebud of her anus. “I guess not. Anyway, I see a girl like you Hannah, all young and pretty and slender, and I all I want to do is go straight to her cute butt and put my cock in that tight little hole. Do I know why that is? What happened to me in my childhood that would give me such a perverted desire as an adult? No idea. But I don’t care either, ha, ha.” He pushed forward and once again penetrated her guts.

“Uhhhhhh!”

“Ah, that’s so good.” He placed both hands on Hannah’s shoulders now and gently slid into her. “I think it’s the best feeling in the world, inside a pretty girl’s shithole. I just want to keep doing it over and over.”

Yes, he just wants to keep doing it over and over. Sometimes characters in stories do stuff that is outside the realms of a normal person's experience. But is it wish fulfillment? Maybe that's a simplistic view. Yeah, that's what it is. But that being the case, where do these fantasies come from? I'd be more than happy to discuss it further. Let me kow, if you like.