After the letter was read in churches Sunday, the Scottish government insisted that it intends to legalize same-sex marriages and religious ceremonies for civil partnerships because "it is the right thing to do."
The issue is still in the consultation stage in England and Wales.
The letter from the Scottish Catholic leadership was part of its latest drive to keep marriage in the province on a traditional path. It called on congregations "to pray for our elected leaders ... that they may be moved to safeguard marriage as it has always been understood, for the good of Scotland and of our society."
Last week, Cardinal Keith O'Brien, who heads the Catholic Church in Scotland, described same-sex marriage as a "grotesque subversion of a universally accepted human right."
At the same time, O'Brien broke off talks on the issue with First Minister Alex Salmond, Scotland's political chief.
The Scottish government Sunday staunchly defended its intention to make gay marriages legal. But it did add that "we are equally committed to protecting religious freedom and freedom of expression," while "ensuring that religious celebrants opposed to same-sex marriage do not have to solemnize same-sex marriage."