There is no nudity. However, many of the story lines revolve around the sex-life of the main characters. Almost every episode references sexuality or sexual activity in some way, including both direct discussions about sex as well as humorous innuendo.
Each character presents their own sexual tendencies or frustrations throughout the life of the show.
Several episodes promote sexual flings, one-night stands, and meaningless sex.
Fay is notorious for using double entendres.
One episode revolves around Joe and Helen trying to be intimate, but keep getting interrupted by Brian and her sister Casey.
Though usually used as a point of humor, sex is also treated with maturity and seriousness when appropriate. Characters deal with homosexuality, monogamy, self-respect, marital affairs, and so on.
Lowell is seen naked in one episode but nothing below his chest is shown.
It is implied that Lowell was having sex with his ex-wife Bunny in one episode, he comes into the kitchen without a shirt and is later pulled back by Bunny.
Casey sees Brian nude twice.
Casey and Brian start developing romantic feelings towards each other despite the fact their siblings are married.
Joe and Helen are seen on their honeymoon constantly making love.
One episode has Joe and Helen walking around their house naked, Joe is shown holding a newspaper and Helen covering herself with an apron.