(Author's Note: I'm trying for a cold opening thing in this one. The next chapter will explain how they got here, but for now, read it as it is and wonder. Maybe discuss it with your book club.)
Dumbledore's shriveled, wizened shaft quivered as he thrust it into the Dark Knight's gaping rectum. "This is not the anus you need, Albus," grumbled Bruce, "but the one you deserve." The bearded old warlock paused a moment, disinserting himself from the multi-milionaire. He pulled a slender stick of wood from a fold in his robes. He pointed it at his organ and said in a commanding, stentorian voice, "Penis engorgibus." His dick became magically shrouded with a larger, silvery double, which he thrust with gusto again into the buttocks of Mr. Wayne. "NNGHNHGHNGHNGHN" growled the superhero, fondling his costume's nipples.
Suddenly, the door opened. A silver tray clattered to the ground. Alfred had walked in on Master Wayne- Master Wayne, who he'd reared almost as a son after those tragic murders- being penetrated by an elderly wizard.
"M-Master Bruce," Alfred gasped, his shaky hand creeping down towards his fly. The elderly butler pulled out his cock, just as shriveled as the wizard's but not quite as large, and began stroking it furiously. Dumbledore, still holding his wand, shot a burst of bright pink light at the butler. Immediately, he began belching slugs all over himself, squirming, mucusy invertebrates covering his body. He was more aroused than ever.
Alfred gathered the slugs toward his schlong, providing him with some much-needed lubrication. This is the closest he has ever gotten to feeling a vagina, as his life as a servant for the Wayne family deprived him of the chance to ever meet a woman.
Meanwhile, Bruce gasped as Dumbledore filled him with more man-mayo than one would think could fit in such a lean man. "Frottage time," he said as he flipped around and whipped out his dick. Albus gripped his and Bruce's cocks, and began stroking them both in unison. He tapped his dickhead with his wand, and it began to writhe and wrap itself around the Bat-dong in a manner similar to Alfred's slugs. The foreskin grew further still: soon all of Bruce's cock and balls were engulfed in Dumbledore's bellend.
Suddenly, the ancient warlock's loose, wrinkly dick-skin began to rotate around Bruce's member. It spun faster and faster until Bruce reached orgasm, at which point the end snapped tight around his genitals. Albus's foreskin filled with Bruce's thick semen as if it was a water balloon.
Dumbledore carefully slid the foreskin off of Bruce's dong and tied a knot in it, securing the baby batter. "There are some potions this is good for," he said absentmindedly to the exhausted multi-millionaire. "I'd work with Snape to get his supply, but he is impotent."
"I'm, hnngh, not surprised, ughhh," grunted Bruce, in ecstasy from his climax, "he always-" Bruce was interrupted by a splattering sound. He and Albus turned around to find Alfred ejaculating buckets of semen onto the floor. Alfred had not masturbated since Bruce's parents died.
"Alfred! For shame. You should have waited a moment for me to get my flask out," scolded Dumbledore, half-seriously. He magicked the semen into a small Mokeskin pouch that didn't look like it could hold the gallons upon gallons that had come from the butler.
"Is that, unghh, a bag of holding, Albus?" asked Alfred, pumping the last bits of semen from his abused shaft.
"Yes, of sorts, but it only works for semen. Us wizards don't really need unlimited space for anything else, so we never bothered to expand on the technology."
"Marvelous," replied Alfred as he went to get a blanket to cover the immobile Batman that lay between them. Suddenly, the form of a muscular man in red-and-blue spandex burst through the window.
"Hey, Bruce!" called the somewhat autistic alien. "You wanna go to Panera Br-Jumpin' Jehosephat! What are you doing?" Superman, of course, has never encountered homosexuality before. "This can all be explained," said Dumbledore to the (apparently fallen) Kryptonian, "over some butterbeer in the Batcave."