Pope Benedict XVI Friday told a group of Catholic bishops in the U.S. to fight same-sex marriage, warning against powerful political and cultural currents legalizing gay nuptials in states around the country.

Sexual differences cannot be dismissed as irrelevant to the definition of marriage, Benedict said in a speech to the U.S. bishops at the Vatican.

Marriage and the family are institutions that must be promoted and defended from every possible misrepresentation of their true nature, the pope said, according to Agence France Presse.

Benedict's remarks came a week after Maryland Gov. Martin O'Malley, a Democrat, signed gay marriage into law. Across the country, same-sex marriage was recently legalized in Washington state. Same-sex marriage in both these states, however, will likely be re-evalauted via referendum before they go into effect.

Currently, there are six states in the U.S. -- New York, Connecticut, New Hampshire, Vermont, Iowa, Massachusetts -- and Washington, D.C., issue marriage licenses to same-sex couples.

Benedict said marriage as a natural institution is rooted in the complementarity of the sexes and oriented to procreation.

Bishops who visited the Vatican for the speech hailed from Minnesota, North Dakota and South Dakota, according to the Catholic News Service.

The speech focused on promoting the Catholic Church's teachings on sexuality. Rejecting a mature sexual ethic based on chastity, the pope said, has led to grave societal problems bearing an immense human and economic cost.