Here is a documentary with an absorbing and unexpectedly complicated story to tell, whose paradoxes and sadnesses are not entirely resolved by the end. Artist and film-maker Rachel Mason has created an affectionate portrait of her elderly parents, Karen and Barry, who in many ways are like one of the (fictional) old couples in When Harry Met Sally.

Karen is a former journalist, devoutly Jewish, and Barry is a former special visual effects engineer who worked on Stanley Kubrick’s 2001 and invented a modification for kidney dialysis machines. But they found themselves in a tough financial spot in the early 1980s and took over Circus of Books, a gay porn bookstore in Los Angeles that also sold movies called things like Confessions of a Two Dick Slut and Don’t Drop the Soap, and was one of Larry Flynt’s first distribution points. Under their shrewd management, the store boomed, opened another branch and became a well-known meeting place for LGBT people, while all the time, the Masons were a conventional family who kept their three children well away from the business. Karen movingly – and honestly – recounts how upset she was to discover that one of her sons was gay: the business and family life were that separate.

Karen moved away from that attitude to become a member of PFLAG, the organisation for LGBT people and their families, but Rachel amusingly shows how, even now, Karen is fastidious about avoiding looking directly at the sex toys she buys for the store from a wholesaler.

There are some things the film doesn’t talk about: Circus of Books was about male images and male consumers, and not really about gay women, and this is not discussed. And the idea of porn equalling sexual liberation is something that has long been challenged by feminist thinking in terms of straight material; is there really no equivalent discussion around exploitation in gay porn? Well, no matter – there is enormous humour and tenderness here.

• Circus of Books is available on Netflix on 22 April.