“Shortbus” begins with a man sitting in his bathtub, videotaping himself as he urinates. What he does after he gets out of the tub, I can’t even tell you.

I can’t tell you most of what goes on in “Shortbus,” in fact, at least not the particulars. It’s a movie about people whose lives revolve around sex — and if you just thought, “Doesn’t EVERYONE’S life revolve around sex?,” then you are definitely the film’s target audience — and it doesn’t shy away from the most intimate details of the characters’ sexual encounters. It revels in them, in fact, not in a salacious way (though what is depicted technically counts as pornography), but in a manner that is joyfully matter-of-fact: This is what these people do, and this is how much they enjoy it.

The writer and director is John Cameron Mitchell (he says the story evolved through workshops with the actors), whose only other film so far is the stellar “Hedwig and the Angry Inch.” If “Hedwig” was a 5 on the eyebrow-raise-o-meter, “Shortbus” is a 50, but they have common themes. “Shortbus,” like “Hedwig,” is about people who fear loneliness.

The setting is New York, depicted in establishing shots as a bright cartoonish landscape where everyone is connected. A variety of pansexual, hedonistic characters representing a variety of psychological types (needy, vulnerable, closed-off, etc.) converge on a sex club called Shortbus, where anything goes and where the only thing required is a positive attitude.

Among them is Sofia (Sook-Yin Lee), a sex therapist — er, “couples counselor,” she politely insists — who has never had an orgasm despite an active sex life with her husband Rob (Raphael Barker). Two of her patients, a gay couple named James (Paul Dawson) and Jamie (PJ DeBoy), are having problems of their own. James was the guy in the bathtub, recently detached and withdrawn from Jamie, who is eager to make James happy but doesn’t know how.

Elsewhere at Shortbus is a dominatrix-for-hire named Severin (Lindsay Beamish), who lives in a storage unit and takes inappropriate Polaroids of strangers on the street. She has difficulty expressing affection, which one gathers is an occupational hazard of being a dominatrix-for-hire.

All of these people (and more in the periphery) mingle with one another as they seek fulfillment. But most ultimately learn that that sort of happiness comes from within, all the enjoyable couplings notwithstanding.

And my heavens, are there ever some couplings (and triplings, and quadruplings)! Mitchell seems hell-bent on making this film entirely unmarketable. I understand his philosophy about honestly depicting sex, but the fact is, most people, even free-thinkers and sexually liberated types, don’t want to see what goes on here, at least not in a public movie theater.

That doesn’t make the film bad, of course; it just makes it unlikely to be profitable. It’s actually a good movie, offering poignant and comedic insight into sex and relationships through some very honest, forthright performances. And it’s certainly a memorable film, I’ll give it that.

B (1 hr., 42 min.; Not Rated, would definitely be NC-17 for abundant graphic sex and nudity, plus a lot of profanity and sexual dialogue.)