Roman Catholic bishops meeting Tuesday in Baltimore have affirmed the church's definition of marriage as a heterosexual union and opposition to gay marriage.

In a pastoral letter by the U.S. Conference of Catholics Bishops issued at the meeting, bishops say it's “just” to oppose gay marriage because it hurts society.

The letter says “it is not unjust to oppose legal recognition of same-sex unions” because such marriages harm society and ignore the roles of husbands and wives.

“To promote and protect marriage as the union of one man and one woman is itself a matter of justice,” the 60-page letter says.

“The legal recognition of same-sex unions poses a multifaceted threat to the very fabric of society, striking at the source from which society and culture come and which they are meant to serve. … By attempting to redefine marriage to include or be made analogous with homosexual partnerships, society is stating that the permanent union of husband and wife, the unique pattern of spousal and familiar love, and the generation of new life are now only of relative importance rather than being fundamental to the existence and well-being of society as a whole.”

Adoption of the letter during the 3-day conference was not a surprise, the church has frequently condemned gay unions. Pope Benedict often speaks out against gay marriage, primarily in Spain, where such nuptials are legal. Last year, the Vatican refused to back a United Nations resolution calling on all countries to decriminalize being gay because they feared it would lead to increased legal recognition of gay unions.