Despite Iran's strict Islamic laws, increasing numbers of young couples are choosing to live together before marriage. It has become so prevalent that the office of the Supreme Leader has issued a statement expressing deep disapproval, as BBC Persian's Rana Rahimpour reports.
"I decided to live with my boyfriend, because I wanted to get to know him better," says Sarah from Tehran.
"It's hard to get to know someone just by going to restaurants and cafes together."
Sarah's decision to enter into what is known in Iran as "white marriage" would have been unthinkable just a few years ago.
In a country where strict Islamic laws mean shaking hands with the opposite sex is illegal, cohabitation is a crime that risks severe punishment.
Nevertheless, increasing numbers of unmarried couples are now choosing to live together.
There are no official statistics, but it has become common enough for a popular women's magazine, Zanan, to devote a special issue to the subject recently.
And now even Iran's Supreme Leader, Ayatollah Khamenei has joined the debate.
At the end of November the head of his office, Mohammad Mohammadi Golpayegani, issued a strongly worded statement calling on officials to "show no mercy" in clamping down on cohabitation.
"It's shameful for a man and a woman to live together without being married," the statement said. "It won't take long for people who've chosen this lifestyle to have wiped out a legitimate generation with an illegitimate one."
But young Iranians do not seem to be listening.
"It's too expensive to get married and even more expensive to get a divorce," says Ali from Tehran, who has been living with his girlfriend for two years.
"Why commit myself to something I'm not sure about?"
It is attitudes like this that show just how far some young urban Iranians have moved away from the Islamic values of their parents' generation.
"Of course cohabitation is not accepted by the more religious parts of society," says sociologist Mehrdad Darvishpour, who is now based in Sweden.
"But just like in the rest of the world, the middle class in Iran is starting to prefer this type of life to traditional marriage. Sex before marriage isn't taboo anymore."
Many observers point to Iran's soaring divorce rates as a key reason why some couples do not want to rush into marriage - and why their families often agree with them.
One in five marriages in Iran ends in divorce, says Farhad Aghtar, Director General of the Office of Drug Abuse Prevention and Treatment, which is part of Iran's State Welfare Organisation. Tehran, the capital, has the highest rate in the whole country.
In Iran it is the groom's family who pay for the wedding, which is often a lavish and costly affair.
The groom also has to put up the money for a dowry or "mahrieh" - a payment to be made to the bride if the marriage breaks down.
Large sums of money are often involved here, too, and men can end up in debt for years after a divorce. Failure to pay can result in imprisonment.
For Iranian women, the prospect of marriage breakdown is also bleak.
Islamic law means it is difficult for women to initiate divorce in the first place. Custody laws automatically favour the father and social stigmas mean life for divorced women is not easy.
Sarah says one reason she decided to live with her current boyfriend was precisely because a previous relationship had broken down when the man's family made it clear they did not think a marriage between the two would last.
"His parents and siblings interfered a lot," she said. "They said I was a loose woman. They put so much pressure on him that in the end he broke up with me."
But choosing to live together is not always an easy option.
Although some urban parents are willing to accept their children's decisions, cohabitation is still considered a step too far by many in what remains a deeply traditional society.
Marjan, who lived with her boyfriend in the city of Arak, says she had to move house four times after landlords found out she and her boyfriend were not married.
"Every day they would ask, 'When are you going to get married? When are you going to buy a ring?'" she said. "I'd think - they're watching me, so I'd try to find another place."
Another problem, according to lawyer and women's rights activist Mehrangiz Kaar, is that because cohabitation is illegal, there is no legal support for couples if things go wrong.
If a woman is abused in a "white marriage", she cannot go to the police, because she and her partner would be arrested for committing adultery, Mrs Kaar said.
This summer, the head of social and cultural affairs at the office of the governor of Tehran, Siavash Shahriar, announced that a plan had been finalised to tackle "white marriage" and, in his words, "to promote family stability". But to date no specific plans have been made public.
Sociologist Mehrdad Darvishpour is sceptical about attempts to use the law to deal with issues affecting people's personal lives.
"What happens under the skin of a society cannot be controlled," he said.
"The government might try to use force to stop this, just as they tried to impose stricter adherence to the rules on wearing a hijab [headscarf] on young women, but young people will continue to move forward. Modernity can't be stopped."