The Plot is Reasonable When You Consider the Air Force Invented Swinging
Maybe Colonel Robert Mitchum and his subordinate's wife was more commonplace a combination then we thought. Even the time period is correct.
From Wiki:
According to Terry Gould's Book The Lifestyle: A Look at the Erotic Rites of Swingers, swinging began among Air Force pilots and their wives during World War II. The mortality rate of pilots was high. Gould reports that a close bond arose between pilots, with the implication that husbands would care for all the wives as their own, emotionally and sexually, if the husbands were away or lost (thus bearing some similarity to levirate marriage).
This is debatable, however, since it would have been unusual for wives to accompany their husbands on foreign tours. Other sources point to U.S. Air Force test pilots in the California desert as the original participants.
Though the beginnings are not agreed upon, it is assumed American swinging began among American military communities in the 1950s. By the time the Korean War ended, swinging had spread from the military to the suburbs. The media dubbed the phenomenon wife-swapping.