Sex is comedy unless the French are making a movie about it. This talky, and boy do they talk and even talk about talking, movie about film making and directing a sex scene in the movie within the movie gets tedious pretty quickly. Since there is little sex and not much comedy (comedy here is meant in the French sense of playing a scene or a role and not comedy as in funny ha ha) you quickly lose interest even if Anne Parillaud (La Femme Nikita) is pretty good as the director trying to convince a recalcitrant Actor and a young ingénue Actress to get busy between the sheets for the crucial scene in her movie.
If you like movies about making a movie, you will probably enjoy Sex Is Comedy. Aside from all the philosophizing about directors, actors, acting, meaning, reality versus fiction, what is real in the world of fake (movies) and so on and so on, you do get a convincing behind the scenes take on the conflict between a director and her actors. Not that this movie is not funny someitimes. There are a couple of almost humorous scenes and ideas, like the one about the prosthetic penis the Actor gets for the nude scene or the fight the Actor has with the director about keeping his socks on before and during the sex scene that, had they been developed could have lightened the mood. There is, however, one funny moment where the Actor, wearing his fake penis, insists on having it dangling out of his robe as he walks around the set. Unfortunately, the movie's director, Catherine Breillat, has the movie's director, Anne Parillaud bitch about it and then leave the room.
If you thought you were going to get a movie about sex, or a comedy, well, blame the French for their usual habit of using cool sounding English words without having a clue what the words really mean. A false cognate is when the same word means two different things in two different languages. For example, comedy: In French, comedy can also mean pretending, as in "Il joue la comedie." meaning he is faking it, bullshiting, lying. This is the real meaning of the title; sex is pretending you are interested in your partner. So, in the French definition of the word, Sex Is Comedy is a comedy: a movie that pretends it is about something it is not. This is perfectly clear in the last scene of this movie where the crucial sex scene in the movie is akin to forced sexual intercourse.