Notes

A bored Saitama decides to visit Bang at his dojo.

Bang was surprised when he heard the knock on the door. Rarely anybody visited his dojo (especially since his only student was already there, exhausted and napping in another room) except the occasional agent from the Hero Association. He was even more surpised to find, at the door, Saitama, one of the strongest, if not the strongest, people on Earth. Had he invited him?
"Hey." Saitama raised his hand in greeting, and Bang nodded as a reply.
"Saitama," the older man said, attempting to not be appear confused, "did I invite you? It seems I've forgotten."
Saitama smiled and shook his head. "Genos is out right now and I was bored, so I decided to come over and see what you were doing."
"I see." In his brief relief that he hadn't forgotten something as simple as an invite, Bang didn't notice both his shirtlessness, and Saitama's slowly increasing lustful gaze at said shirtlessness. "I'm afraid, however, you came to the wrong place. Exciting things rarely happen here."
"That's fine," Saitama replied, his gaze still fixated on Bang's well-muscled abdomen as the martial artist continued to converse unaware of the stare.
"Would you, perhaps, be interested in learning more about the Water Stream Rock Smashing Fist?" Bang tilted his head slightly, but Saitama simply shook his head.
"No," he said, "I'm actually much more interested in something else."
And that was how Bang found himself naked on one of the mats in his dojo, his nipples peaked as they were mercilessly teased by rubber-glove covered hands and his cock leaking pre-come all over the floor.
"S-Saitama!" Bang cried out as Saitama grasped a nipple and twisted it slightly. He then gasped as the younger grasped his member and rubbed at the head, sobbing slightly at the overload of pleasure flooding his senses.
"You really shouldn't answer the door," Saitama said genially as he continued to tease Bang, "looking like that, Bang. You're just asking to be fucked."
Absently, Bang prayed to whatever deity was there to listen that Charanko wouldn't wake up anytime soon as he continued to writhe helplessly under Saitama. He tried not to whine as hands left his nipples, only for the stifled sound to turn into another yelp as one hand took his member and forced more pre-come out of it and the other grabbed at his ass, sticking a finger unashamedly into his hole.
"Now, let's see," Saitama muttered, his gaze shifting to Bang's hole as his right hand absentmindedly stroked Bang. The older man suddenly screamed (literally screamed) as something was pressed in him that sent overloading waves of pleasure across his entire body. He arched in the air and whimpered helplessly as his chest was enveloped in a warm heat - Saitama's mouth. His ass lifted away from Saitama's invading hand, but he simply followed its movement, pressing against his prostate unapologetically as his other hand wrapped around the head of Bang's cock, slick with pre-come, and gathered it around the tip.
Suddenly, Bang screamed louder than ever before and his back shot off the floor, his cock straining against his abs as ropes of come shot across his muscular body. Saitama watched with a sort of amused boredom, and, after half a minute, when Bang collapsed against the floor, his body shining with sweat, he resumed his activities much to Bang's protest.
"Pl-please!" Bang cried out. "Stop. Sai-SAITAMA!" Saitama simply raised an eyebrow and, in reply to his pleads, pressed his three fingers stuffed firmly in Bang's ass harshly against his prostate, grinding against it until the old man was coming for the fourth time in two hours.
"You're too fucking hot, y'know that?" Saitama muttered, almost to himself, as his hands continued to play with Bang's body, now attuned to his sensitive spots and exploiting them mercilessly. His hands slid across smooth muscles, every once in a while pressed into Bang's ass and milked him further, or teased his cock until Bang was boneless, twitching and gasping and moaning and incapable of moving. And even still Saitama continued, until he eventually got bored 10 hours and 43 orgasms in, and finally decided Bang was ready to recieve his cock.
The old man, under a hazy spell of pleasure, barely registered it as the slick cock pressed against his hole, moaning helplessly as his limp cock twitched slightly from the unwanted stimulation. And then, before he knew it, he was screaming himself hoarse once again as Saitama's member pressed against his prostate, sliding against it and bringing Bang to another forced orgasm. His body, covered in come and sweat, collapsed against Saitama's still clothed body, and he moaned slightly as hands went to his chest and swirled the come around his muscled form, continuing to tease and torment his body until it was eased into his 45th orgasm.
"You're so fucking hot." Saitama moaned as he stared appreciatively at Bang's well-built form covered in come. "I should bring Genos along and ask him to bring a camera. I'll fuck you to a hundred orgasms as you lay there, helpless, watching yourself come and come over and over again whilst unable to control your own body."
Bang could only moan.
And so it came to be that Silver Fang became Bang the Slut. Eventually introduced to the public as the Hero Association's slut with Saitama as his primary master, Bang went from a respected hero to the cum dumpster of the S-Rank Heroes.