As debate over health care coverage for birth control rages in the US, Benedict said there was an urgent need for Catholics in America to discover the value of chastity — an essential element of Christian teaching that he said had been subject to unjust “ridicule”.

Benedict has long championed traditional marriage and fidelity but his strong comments to visiting US bishops took on particular significance given the culture wars in the US.

US bishops are currently locked in an election year battle with the Obama administration over federal funding for birth control.

In addition, bishops have been at the forefront of the campaign against same-sex marriage, with at least five US states scheduling ballot measures on the issue.

Cardinal Timothy Dolan, head of the US conference of bishops and archbishop of New York, unsuccessfully lobbied against the legalisation of gay marriage in his state.

The 84-year-old Pope acknowledged his comments might sound anachronistic or “counter-cultural”, particularly to the young. But he told bishops not to back down in the face of “powerful political and cultural currents seeking to alter the legal definition of marriage”.

“Sexual differences cannot be dismissed as irrelevant to the definition of marriage,” he said.

He also denounced what he called the failure of priests and bishops to instruct Catholics in core Church teachings on human sexuality, saying many Catholics seem unaware that living together outside of marriage was “gravely sinful, not to mention damaging to the stability of society”. The entire Christian community, he said, must recover an appreciation of the virtue of chastity.

“Young people need to encounter the Church’s teaching in all its integrity, challenging and counter -cultural as that teaching may be; more importantly they need to see it embodied by faithful married couples who bear convincing witness to truth,” he said.

Benedict said a weakened appreciation for traditional marriage and widespread rejection of responsible sexuality had led to “grave social problems bearing an immense human and economic cost.”