“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 countercultural 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 the widespread rejection of responsible sexuality had led to “grave social problems bearing an immense human and economic cost.” He didn’t elaborate on what the cost was.

Groups of visiting U.S. bishops have been traveling to Rome for the past several months as part of bishops’ regular once-every-five-year visits with the pope and senior Vatican officials.