Pope Benedict XVI urged visiting U.S. Catholic bishops to take on America's culture of sexual permissiveness.
In a meeting at the Vatican, Benedict, 84, encouraged the bishops to strengthen church teachings against premarital sex and cohabitation.
He said U.S. Catholics need to rediscover the value of chastity.
"Young people need to encounter the church's teaching in all its integrity, challenging and counter-cultural as that teaching may be," Pope Benedict said.
"More importantly, they need to see it embodied by faithful married couples who bear convincing witness to truth," he said.
The Pope also condemned the "powerful" gay marriage lobby in America, admonishing the U.S. bishops to stand up against the "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.
Meanwhile, the Catholic Church in England is stepping up a campaign to stop the government from legalizing same-sex marriage there.
Priests in 2,500 parish churches read a letter from archbishops saying the government's plan will weaken the institution of marriage.