Why Not! (1977)

A threesome becomes a foursome in this sensitive drama. The tale begins with the relationship between a recently divorced man and woman (from different marriages) and the bisexual they get involved with. At first all three are happy in their new arrangement, but then the divorced fellow suddenly leaves and those remaining in the relationship become quite tense. Fortunately the fellow returns with another, more conventional fellow. Eventually the three persuade him to join them.