Menage a Quatre: Thirty-Six Hours of Pleasure — Rex Gordon

When two couples meet by chance during a weekend in Paris.  A bored husband makes an audatious suggestion.  The beginning will be spouse swapping, with each partner making love to the other's husband or wife.  But soon the four will find they are embarked, not just on a voyage of sexual discovery, but of personal discovery as well.  While they try out every possible comination, twos, threes, same sex and four in a bed, they are also learning things about themselves they never suspected.  What they do and what they learn during their thirty-six hours of pleasure will change their lives forever!