“Again,” by Ramsey Campbell
Originally appeared in Rod Serling’s The Twilight Zone Magazine (which ran from 1981 to 1989); featured in PseudoPod 550, July 7th, 2017; apparently anthologized all over the place, including in Vile Things from Comet Press
Maybe 5000 words?
A truly creepy story, repellent and fascinating. It gets a lot of its force, I can’t help but feel, from the reader’s discomfort with kinky sex and with old people having sex lives at all.