In the privacy of their bedrooms in conservative Iran, Sabah's husband used to burn her with cigarettes during sex, while Sarah was forced to re-enact scenes from the sadomasochist box office hit "Fifty Shades of Grey."

The two women are among a rising number of Iranians getting divorced, a trend that some experts attribute to women's growing confidence in speaking out about violent sex and marital rape.

"He was obsessed with the movie 'Fifty Shades of Grey' and tried to emulate the sexual methods of characters by using handcuffs and a mask," said Sarah, 29, who was married in the capital, Tehran, five years ago.

Both women declined to give their full names.

Divorce has doubled in Iran over the last decade, angering conservatives who see the 164,000 cases recorded in 2015 as an affront to the values of the Islamic Republic and worrying authorities as the birth rate is also falling. Experts point to two main drivers: More men are learning about sex through porn and imitating violent scenarios, and Iran's increasingly educated, financially independent women are less willing to put up with it.

"My husband used to have sex with me like an animal," said one London-based Iranian divorcee, whose husband raped her on their first night together.

"My baby died in the womb due to his manhandling, since I was not consenting to his sexual appetite."

Sex slaves

Presidential spokeswoman Shahindokht Molaverdi recently told the Iranian Students News Agency that "sexual dissatisfaction is the main cause of divorce in Iran."

She said the government has set up family health clinics nationwide to counsel couples about their sex lives, especially when they are considering divorce.

Mehrdad Darvishpour, a sociology lecturer in Sweden's Malardalen University, believes the Shi'ite-majority country needs to start talking about sex more openly as most men are unaware of women's needs with no sex education in schools.

"The only source of information [men] have is porn movies, which portray women as a sexual tool," he said. "They expect the same in their lives, by treating their wives as sex slaves."

Iran censors the internet, banning thousands of websites with content it regards as unIslamic, but experts say it is easy to buy pornographic videos in local markets.

Pre-marital sex is also illegal. Sarah didn't find out about her educated husband's sexual sadism until it was too late.

"Most nights he preferred watching porn movies and then imitating them," she told the Thomson Reuters Foundation by phone.

Disobedient

Giti Pourfazel, a human rights lawyer who practiced law for about 25 years in Tehran until 2016, estimates half of divorces in Iran are due to sexual problems.

Her female clients often complained about their husbands' violence during sex, while men got angry that their wives refused to sleep with them, she said in a phone interview.

Iranian law favors the husband, who has the right to ask for a divorce. But if he is unwilling to divorce, the wife must legally prove that he is abusive, has psychological problems or is somehow unable to uphold his marriage responsibilities.

It took Sabah, whose marriage was arranged by her family, 10 years to escape the cigarette burns and violence.

"The ultra-conservative society of Khosistan considers divorce a disgrace," the 37-year-old said, referring to the southern province where she lives.

She finally succeeded by pushing for payment of her mehrieh, or dowry, which was not paid when the couple married.

Dowries in Iran have skyrocketed to tens of thousands of dollars and wives can sometimes waive outstanding payments as part of a separation settlement.

For the London-based woman, life after divorce has been a revelation.

"During those distressing years with my husband, I forgot my desires and developed negative thought about sex," she said.

"Now at the age of 50, I realised I have sexual desire and I came into a beautiful relationship here in the U.K. with a man."