"Whoa... What happened to you, little guy?" Clay asked, tossing the once-larger man the towel.
Adam didn't even move, he just lay back and moaned, his hands cupping his oversized balls, tenderly. Clay rolled his eyes and hunched down on the balls of his feet looking over Adam's still-soaked body. He felt himself stirring and looked down at his own, tiny cock as it got hard.
Slowly, realization dawned on him and he glanced from his dick to Adam to the desk where the quarter-inch-tall Mario was back to himself. "So that's it..." he said, slowly. "Your cum is what's shrinking things and I must've gotten some on my dick..."
Standing, Adam took a few steps back and looked down at Adam as he slowly got smaller. The physics student still seemed to be a bit winded from his several-minute orgasm. "Gods..." muttered Clay, annoyed. "Ok, here this: I wish Adam wouldn't get worn out by orgasming and that it would feel good for him to cum, no matter how much he did it..."
A sigh of relief escaped Adam's lips as he felt a wave of relief wash over him. Leaning back, he partially lay on the towel as Clay looked down at him. The once large man had diminished quite a bit now, his body pulling in on itself as it dwindled past four-foot-six to four-foot-five ... four ... three ... two...
He was losing almost an inch every couple seconds. Clay grinning. "Mmmmm... Looks like you'll be joining Mario pretty soon..." he said. Glancing between the bigger man's legs, he noticed that while his balls and body had indeed shrunk, his cock was still the same, four-inch size it had been, before.
Clay slapped his forehead. "Oh, I get it! Your cock will always be the same size as mine, now... No matter how small you get, right?"
Adam finally got enough breath back to look up at the new roommate with a glare. "Look, Clay... This isn't funny..."
"Seems pretty funny to me, small-fry..." Looking down at the disproportinately big penis between Adam's legs, Clay chuckled. "But you're right. Since your cum shrank my cock, I don't know if I'll ever get beyond four inches again unless I somehow put a wish on you that makes it happen..."