You can possibly see a shadow of a girl's nipple as she's being bathed by a doctor. Done rather sexually.
Elaine says "I wanna slide my tongue around you like a snake" as a joke. The whole episode revolves around the fact that there is a woman recording sexy and erotic messages on Jerry's tape recorder.
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In the end of one episode, we can see Newman and Kramer's mom briefly having sex.
George jokes about how a story must be good or else it's "simply just masturbation."
An entire plot is how the main characters have a contest to see who can go the longest without masturbating, although the word "masturbate" or one of its variations is never used. A nude woman across the street from one of the main character's apartments makes someone the first to lose the contest. The woman is never shown, though. References are made to JFK's "butt."
An entire sub-plot revolving around orgasms, including multiple uses of the word "orgasm."
Rachel sees George naked and giggles and he tries to convince her that it was just temporary shrinkage because he had been in the pool.
A sex scene. All of the action is offscreen, but lots of moaning and heavy breathing can be heard.
There are many references to birth control and sponge-worthy. In one scene George is seen trying to open a condom.
George tries to combine food, sex and tv and there are many references to it.
Another one revolves around the word "ass" (see profanity). Some pretty heavy kissing scenes, occasional implied sex and nudity.