OK, I think I found it. The story is called "The White Beard," and in it Buk's character and two coworkers encounter a "13 or 14 year old girl" with big breasts. Buk has sex with her, and then the other coworker does too. Wen he comes down from her room:
"I rammed it between her breasts," said Herb. "Then a sea of come under her chin. When she stood up it hung there like a white beard. She needed two towels to mop it up."
It's on page 231 of ""Erections, Ejaculations, Exhibitions, and General Tales of Ordinary Madness. Hope this is what you were looking for.