It's a friend of mine who wants to know, so I said I'd consult MN, where you can get an answer to anything...
It was called something like A-One, and published in an anthology of short stories some time before 1976. It's about a girl/young woman who lands on an alien planet and moves in with a couple, and gradually discovers that they swap sexes to breed. My best guess was The Left Hand Of Darkness but she says it's not, and anyway that was a novel. Does it ring any bells with anyone?