On Monday, the Department for Education released its long-awaited guidance on how schools should teach teenagers about sex, to much fanfare.

The updated rules, which come into effect in September, mark the first time the government has overhauled relationship and sex education (RSE) in almost 20 years.

News that pupils will now be taught about periods, female genital mutilation, safety online, and lesbian, gay, bisexual and transgender relationships has sparked a flurry of excited headlines.

But one key issue is still very much up in the air: when, and how, should parents be able to withdraw their children from these nominally compulsory lessons?

The new guidance says parents can choose to opt their child out of RSE up to age 15 and “except in exceptional circumstances” the school should let them.

Pressed repeatedly to explain what those circumstances might be, Damian Hinds resorted to debate-club sophistry, saying that was impossible as, “by definition, they are exceptional”.

“There is a balance to be struck here and I think we have struck it,” the education secretary told Parliament.

Others disagreed. Tory MP Sir Edward Leigh described the move as “a fundamental shift of power to the state” that went against fundamental Conservative values.

Matthew Offord, MP for Hendon, pointed to “huge amounts of concern” in his constituency, adding: “The last time I looked, the Conservative Party believed in freedom of choice.”

‘Morally fraught’

In many ways this question strikes at the heart of the UK’s unease when it comes to young people having sex.

We know they are having it. Despite a recent fall in teen birth rates, we still have the highest rate in Western Europe, at 6.2 per cent - nearly four times as much as Italy, the Netherlands and Sweden.

We know they are talking about it. Numerous studies tell us of the sexual harassment pupils face on a daily basis, both at school and online.

Even GCSE science textbooks feature diagrams of pregnant women with pubic hairstyles that would once only have been seen in porn films.

Yet, the idea of teaching young people how to navigate sex and relationships in the classroom, in the same way we do other important life skills, remains morally fraught.

That much was apparent on Monday when, as Mr Hinds was addressing the Commons, other MPs were debating whether parents had a “fundamental right” to withdraw their children from sex ed.

By leaving the question of when schools should be allowed to go against parents’ wishes so open to interpretation, the DfE has, in effect, made them the gatekeepers of social values.

The new guidelines are without doubt a big step forward, but the onus will still be on teachers to make them work.