Sex is gross. It's visually gross, anyway, clumsy and unaesthetic to those not involved. No wonder filmmakers typically use either dance or exercise as a metaphor. The actual getting-down, the sweaty, repetitive craziness of it, is on some level visually offensive to the non-turned on--or at least those who pretend not to be.

People who chuckled at the dirtiness of the ass-to-mouth jokes in Clerks 2 will choke on John Cameron Mitchell's newest film, Shortbus, where it actually happens. If they spent the year snickering over Brokeback Mountain, they'll walk out by the titles. Shortbus has explicit sex: recreational, polymorphous, homo and hetero sex.

The title comes from the nickname of a bus that ferries "special" children to school. John Cameron Mitchell, the star, creator and director of Hedwig and the Angry Inch, spent several years assembling an improvisatory cast of newcomers culled from an open online posting, and then worked with cast members to see how far they could go on camera. Consciously or not, Mitchell has combined the plot of two of the best-loved porn movies, Behind the Green Door and Deep Throat.

Set in a post-9-11, Bush-ruined Manhattan, a sex counselor named Sofia (Sook-Yin Lee) has been faking satisfaction ever since she was married. Still, she has a ridiculously athletic sex life; at one point, she's braced on the keyboard of a piano. "I feel sorry for people who can't have what we have," she says, lying to her husband with a wide, scary smile. If you wanted someone to defuse a frightening subject like sex, Lee is the perfect actress: small, herbivorous-looking and embodying a Canadian gentleness (where she's a TV personality).

Sofia tearfully blurts out the truth to two of her patients, a pair of unhappy gay clones named James and Jamie (Paul Dawson II and Paul DeBoy). They urge her to attend the illegal Shortbus salon--a sex cabaret replete with orgy room--in a forbidden part of town. There she meets someone to whom we've already been introduced: an ornery dominatrix named Severin (Lindsay Beamish) who has a high-rise office overlooking the bloody stump of the World Trade Center. Severin tries to therapize the therapist and, unfortunately, Sofia is just as liable as to fall victim to a false epiphany as any of her clients.

James and Jamie attempt to spice up their love life by taking on a third partner, but the new man finds interest in the two of them only as a set--they seem like such a perfect couple. Jamie is aching to set out on his own, one way or another. (The two had been previously known as "James and James"--changing his name was Jamie's first step on the road out.)

The visitors at Shortbus help provide support for these crises. Justin Bond, a well-known Manhattan cabaret figure who typically performs under the name of Kiki, has the role of brothel madame, but the press notes assure that he plays himself. Our cinema is loaded with witty gay male quippers, but Bond is the only genuinely funny one I've seen in a long time. He gives depth a holiday.

Shortbus is a lovable film for neither over-intellectualizing or over-dramatizing its plights, and movies with sex are traditionally held together with drama instead of comedy. Mitchell has larded the film with smart jokes: a plaque reading "New York Sensory Deprivation Center--Fourth Floor"; an overdose victim comes to in "Our Lady of Adequate Mercy Hospital." Still, Shortbus has a center. In showing the numbness of a pair of characters who are calloused by sex work, Mitchell reflects the cruel, conformist and deadening world of mainstream porn.

Shortbus is held together by shots of a cardboard and tempera city over which a camera gyres and gimbals. Compare the unthreatening kid stuff of Michel Gondry's sets in The Science of Sleep to Mitchell's cardboard Manhattan, a maze of buildings around the thick and dangerous forest of Central Park. It's alive with sexual energy, but is plagued by brownouts--misunderstandings causing a disturbance in the sexual force.

The wall between porn and non-porn will come down. Filmmakers as different as Bernardo Bertolucci, Lars von Trier, Wayne Wang and Catherine Breillat have made holes in it already. John Cameron Mitchell's tart, sweet but never sloppy film shows its actors actually having sex, putting it ahead of the pack to come.

Shortbus is not the work of a fraud, a pornographer pretending to be an artist. Sometimes the actors fail, some situations aren't as compelling as others, and Mitchell has trouble wrapping his film up. Still, Shortbus is everything an underground movie ought to be. It's a joy, it's a threat to the established order and it's a celebration of messy urban life and what Mitchell describes as "permeability"--the ability to let ideas and other people through the armor.

'Shortbus' opens Friday, Oct. 13, at the Rialto Lakeside Cinemas, 551 Summerfield Road, Santa Rosa. 707.525.4840.

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