Don’t you just hate it when a film confounds your expectations? The teenage son of lesbians Nic and Jules is baffled by his discovery that their taste in late-night movies runs to gay male porn rather than hot girl-on-girl action. Hopefully, British audiences won’t be disappointed that Lisa Cholodenko’s The Kids Are All Right turns out to be a bright, well-observed comedy about family dynamics rather than another titillating romp through the lesbian dating scene.

Nic (Annette Bening) a hard-working doctor, is happily married to Jules (Julianne Moore), whose chequered career path is now branching off into landscape gardening. Nic is the biological mother of 18-year-old Joni (Mia Wasikowska) and Jules’s son is the bizarrely-named Laser (Josh Hutcherson). Both kids are the fruit of the same sperm donor, and when they decide it’s time to meet him, things get a whole lot more complicated – for everyone.

“Donor dad” Paul (played by Mark Ruffalo) is an organic gardener and restaurateur who punctuates his sentences with “cool” and seems to attract leggy babes into his bed without even trying. He’s excited by the idea that these poised and attractive teenagers — brainy blonde Mia and high-school jock Laser — share his genes. But successfully marketing your sperm does not make you a real father.

The script, by Cholodenko and Stuart Blumberg is the product of several years’ work – and it shows. Everyone thinks they’re an expert on families, but creating a setup that doesn’t rely on excessive sentimentality, broad caricature or Wes Anderson levels of dysfunction is harder than it looks. In Bening and Moore, Cholodenko has cast two accomplished actresses with the maturity, the emotional intelligence and the range to convey all the confusion of a mid-life crisis and a soon-to-be empty nest. The fact that they’re gay is – if you can believe it – not the central issue here.

Moore’s record in this genre has been patchy – relying too heavily on klutzy physical comedy and lame romantic set-ups with the likes of David Duchovny. Her bra-less, hippy chick persona here feels like a departure – a good one – from the angst-ridden wives she’s played to such acclaim. The speech to a dumbstruck Laser about the merits of male versus female porn hits exactly the right note – it’s endearingly earnest yet totally unselfconscious.

There’s a similar lack of inhibition about Moore’s Jules when her landscaping assignment in Paul’s garden takes an unexpected turn. (Well it wouldn’t be remotely unexpected in a straight rom-com.) One minute they’re discovering a mutual appreciation of the word “fecund”, the next they are engaging in some truly eye-opening bedroom acrobatics.

Ruffalo, whose super-chilled stoner dude delivery is reminiscent of “the Dude” himself, Jeff Bridges, feels like perfect casting in a film that delivers a sunny, Southern California vibe right from the opening credits. Some might feel his role here is too obviously pitched as the clueless single guy blundering into a family set-up he doesn’t really understand. But Hutcherson’s Laser is smart enough to size him up as someone who isn’t an ideal role model.

Cholodenko’s earlier California-set comedy, Laurel Canyon (2002), foundered because it lacked likeable or believable characters. In Bening’s Nic, The Kids Are All Right has the movie’s most grounded and thoughtful person – though Wasikowska’s Joni runs her a close second. As anxieties about Paul’s presence start to gnaw at her, Nic increasingly takes refuge in red wine and a well-timed restaurant rant about the Californian propensity for fetishising food. It’s a reminder that Cholodenko can do satire, as in High Art’s expert lampooning of pretentious New York magazine publishers.

Bening’s best scene comes during a dinner party at Paul’s house, when she belatedly tries to bond with him over rare steaks and a mutual appreciation of Joni Mitchell’s Blue. As she launches into her own rendition of “All I Want”, she’s vulnerable, lost in the moment and oblivious to the kids’ embarrassment. The audience knows that at any moment she could discover a clue to what’s really been going on during the “gardening” sessions.

Cholodenko has described her movie as “pro family” and – despite the lesbian set-up – it does feel like a very traditional comedy. Costing around $4 million, this film is both low-budget and refreshingly low concept – proving you don’t need outlandish plotlines and a smorgasbord of special effects to attract audiences. I loved it. If only we could have The Kids Are Still All Right in 2011 instead of The Hangover 2.