Gay father - Jerry - lesbian mother - Suzanne - are both carrying out clandestine affairs. But the man Jerry is with is not gay, only bisexual, and is cheating on Jerry with the Suzanne's girlfriend. So you have this kind of weird trapezius of illicit love. Oh, neither Jerry nor Suzanne knows about each others homosexuality.
They do have a child, named Sam (mmm, nice asexual name) but he is gender confused and cross dresses, much to his parent's consternation. When he is around his mother, he wears a dress and she calls him "Sammy," and when he's around his father, he wears boy clothes and is called "Sam".
Slowly, the plot unravels as Jerry and Suzanne find out about each other's lovers and each other's sexual orientation. Then you find out that Samuel was ridiculed throughout his years in Elementary, Middle and High School due to his parents orientation, which the entire town seemed to know about, or at least it was a piece of choice gossip. After witnessing his parents first hand with their partners, he becomes confused and kills himself to avoid the humiliation of his parents action.
Both Jerry and Suzanne have blocked his death, and have been hallucinating Samuel, who was their interior justification for continuing the affairs. They both recall his death and the horror of his suicide, and break down.
They find out about each other, and decide to get a divorce. They begin to sign the papers at the dinner table, with the empty chair that Samuel used to occupy between them.