Despite its title, A Midsummer Night's Sex Comedy keeps its images and language within the bounds of a PG rating. Still, sex and comedy are the only things on its mind, making this the most lightweight Woody Allen movie since before ''Annie Hall.''
The film is set at the turn of the century, and the plot owes a nod to the brilliant ''Smiles of a Summer Night,'' by Ingmar Bergman, whom Allen deeply respects. The characters - three men and three women - gather at a house near an ''enchanted forest'' and flirt relentlessly for an hour and a half. Along the way, Allen satirizes period films and old-fashioned attitudes, yet allows his characters and their epoch to keep a fair share of dignity, lending a handful of ballast to the otherwise frivolous proceedings.
For a film directed by Woody Allen, who also wrote the script and plays a leading role, this ''Comedy'' has surprisingly few laughs. This apparently doesn't bother Allen, who seems less concerned with guffaws than with building a buoyantly romantic mood. Here he succeeds well, thanks to his skill with the camera, and to the exquisite cinematography of Gordon Willis, whose work is luminous from first frame to last. And also thanks to Felix Mendelssohn, whose music provides a gusto similar to Prokofiev's in ''Love and Death'' and Gershwin's in ''Manhattan.''
Allen plays his role - a stockbroker-cum-inventor - within his usual small but effective range, ably backed by Tony Roberts, Mary Steenburgen, Mia Farrow, Julie Hagerty, and Jose Ferrer at the top of his form. It's a minor movie, with no more substance than the ''spirits'' who haunt its woody surroundings. While it will please Allen's ardent fans, it misses the complexity that has marked his work for the past several years. Since he reportedly has another movie due at the end of the year, it will be fascinating to see whether this ''Comedy'' is a deliberate step back toward thematic simplicity, or a holding action to appease his admirers while he prepares another magnum opus.