FICTION : MY HARD BARGAIN <i> by Walter Kirn (Knopf: $18.95; 145 pp.)</i>

In the very first of these 13 short stories, Walter Kirn proves himself to be a tactful writer. You have to have tact to write a story about masturbation without embarrassing your readers, and Kirn--who was raised on a Minnesota farm and educated at Princeton and Oxford--keeps a very straight face indeed.

The coach of a Mormon church basketball team comes up with a plan to cure his players of self-abuse. He gives them poster paper and invisible-ink markers and tells them to record each sin with an X. A month later, at a team meeting, he hangs up the posters and turns on a black light “so that you can see yourselves the way God sees you.”

The youthful narrator expects to be shamed; instead, he is awed by “our X’s emerging out of the wall . . . in bands and columns. . . . Our sins were yellow stars that ran together. I wanted to stay there all night, just looking.”

In this and several other stories about Mormons, Kirn manages to write as both insider and outsider. One teen-ager watches approvingly as a team of missionaries converts his malaise-ridden family. Another youth, the chaperon of a girl whose boyfriend is serving as a missionary in Korea, lets her take him to bed after the missionary, now an ascetic and a martial-arts freak, returns and spurns her.

Kirn writes of idled land and dying animals, of people who don’t win or lose so much as simply hang on with a measure of dignity. Some of his stories are understated to the point of flatness. But others--such as the one about an old man who tries to recruit a wife for his son from among motorists marooned by accidents on a “devil of a curve” near his house--hint that even the straightest face can hide a taste for mischief.