Pope Benedict XVI has called upon the Church to defend marriage in a meeting with US bishops.

The address came at the end of the prelates’ ad limina visit to the Vatican, during which they must give an account to the Pope of the state of affairs in their dioceses.

Speaking in English, the Pope raised the issue of the “contemporary crisis of marriage and the family”.

“It is in fact increasingly evident that a weakened appreciation of the indissolubility of the marriage covenant, and the widespread rejection of a responsible, mature sexual ethic grounded in the practice of chastity, have led to grave societal problems bearing an immense human and economic cost,” he said.

The Pope said there were “powerful political and cultural currents” seeking to alter the legal definition of marriage.

“The Church’s conscientious effort to resist this pressure calls for a reasoned defence of marriage as a natural institution consisting of a specific communion of persons, essentially rooted in the complementarity of the sexes and oriented to procreation,” he said.

“Sexual differences cannot be dismissed as irrelevant to the definition of marriage. Defending the institution of marriage as a social reality is ultimately a question of justice, since it entails safeguarding the good of the entire human community and the rights of parents and children alike.”

The Pope’s comments come a week after Maryland became the eighth state in the US to legalise gay marriage.

The Bill legalising gay marriage states that religious officials will not be forced to perform marriage ceremonies that are “in violation of the Constitutional right to free exercise of religion”.

The Maryland Marriage Alliance, the Maryland Catholic Conference, and the Maryland Family Alliance are planning to sponsor a referendum in November to maintain the definition of marriage between a man and a woman.

The first same-sex marriage licences will not be issued in the state until 2013.

In the UK, the Government is launching a consultation this month on widening the definition of marriage to same-sex couples.

A petition was launched in support of the current legal definition last month and has gained more than 120,000 signatures.

The Pope said children had the “fundamental right” to grow up with a “healthy understanding of sexuality and its proper place in human relationships”.

“It is not merely a question of presenting arguments, but of appealing to an integrated, consistent and uplifting vision of human sexuality,” he said.

“The richness of this vision is more sound and appealing than the permissive ideologies exalted in some quarters.”

He went on to say that there was an “urgent need” for the “entire” Christian community to “recover an appreciation of the virtue of chastity”.

The cohabitation of many young couples posed a “serious pastoral problem”, he said, with couples often unaware that it is “gravely sinful, not to mention damaging to the stability of society”.