By Philip Pullella

VATICAN CITY (Reuters) - Pope Benedict on Friday denounced the "powerful political and cultural currents" seeking to legalize gay marriage in the United States, where Maryland has just become the eighth state to allow it.

The pope's latest comments in opposition to homosexual marriage came in an address to bishops from several Midwestern states on a regular visit to the Vatican.

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

He added that the traditional family and marriage had to be "defended from every possible misrepresentation of their true nature" because, he said, whatever injured families injured society.

"In this regard, particular mention must be made of the powerful political and cultural currents seeking to alter the legal definition of marriage (in the United States)," he added in a clear reference to gay marriage.

Last week Maryland legalized same-sex marriage.

Massachusetts, Iowa, Vermont, New Hampshire, Connecticut, New York and the District of Columbia currently allow gay and lesbian weddings.

Washington State will join the list in June unless opponents stop it ahead of a possible referendum, and Maryland will be added in January 2013 unless its law, too, is overturned by a threatened referendum in November.

Benedict called on American bishops to continue their "defense of marriage as a natural institution consisting of a specific communion of persons, essentially rooted in the complementarities of the sexes and oriented to procreation".

The Vatican and Catholic officials around the world have protested against moves to legalize gay marriage in Europe and other developed parts of the world.

NEW CARDINAL LEADING OPPONENT

One leading opponent of gay marriage in the United States is New York Archbishop Timothy Dolan, who was elevated to cardinal last month.

Dolan fought against gay marriage before it became legal in New York state last June, and in September he sent a letter to President Barack Obama criticizing his administration's decision not to support a federal ban on gay marriage.

In that letter Dolan, who also holds the powerful post of president of the U.S. Bishops' Conference, said such a policy could "precipitate a national conflict between Church and state of enormous proportions".

The Roman Catholic Church, which has some 1.3 billion members worldwide, teaches that while homosexual tendencies are not sinful, homosexual acts are, and that children should grow up in a traditional family with a mother and a father.

Gay marriage is legal in a number of European countries, including Spain and the Netherlands.

Some other Christian Churches that have allowed gay marriage, women priests, gay clergy and gay bishops have been losing members to Catholicism, and the Vatican has taken steps to facilitate their conversion.

While still controversial in the United States, same-sex marriage has been gaining acceptance recently. New Jersey passed a gay marriage law through both legislative houses, though the legislation was vetoed by Republican Governor Chris Christie.

An appeals court overturned California's ban on gay marriage, enacted through a 2008 referendum.

(Additional reporting By Alice Popovici in Annapolis, Maryland; Editing by Alessandra Rizzo)