"If anal turns you on, you are definitely not alone," Gwyneth Paltrow states in her latest blog on her Goop website. The Hollywood actress, whose healthy lifestyle is often the subject of ridicule, is raising eyebrows with her latest offering – a guide to the once taboo sex act.

"First it was shocking, then it was having a cultural moment, now it's practically standard in the modern bedroom repertoire – or so a quick scan of any media, from porn to HBO, will tell you," she writes.

Written in the form of a Q&A with psychoanalyst and author of book The Guide to Getting it On!, Paul Joannides, Psy.D, Paltrow aims to shed some light on the light on the act reminding readers that "the way you see anal sex portrayed in porn is about as real as how they drive cars in The Fast and the Furious."

"The book is amazing not just for its straight-up factual information on practically any aspect of sex you can think of, but also for its easy, nonjudgmental, at-times humourous tone" she writes.

However, she adds: "Its prevalence doesn't change the fact that it's the riskiest sexual behavior in terms of HIV and other STDs."

Joannides notes that between 10% to 15% of all straight couples enjoy anal sex, and around 50% of gay men.

According to Joannides, the line between the anus and vagina became blurred once the porn industry moved online and there was a need for porn producers to add shock value. "The anus isn't designed to have a penis thrust up it; nature did not spec it to handle incoming, as she did with the vagina."

Joannides advises that parents might want to hold off discussing anal sex with children during the 'birds and bees' talk. "I don't see anal being at the top of most parents' "'should talk to our kids about' lists. There are more important things we need to be talking about first."

The anal guide is part of, latest Goop The Sex Issue in which the actress also tackles female orgasm, casual sex, toys and dating over 40.