The professor of moral theology at St Patrick's seminary in Maynooth, Fr Vincent Twomey, has said Catholics cannot vote for same-sex unions.
"A Catholic citizen or politician cannot vote for so-called 'civil unions' between people of the same sex," he said.
"To ask a Catholic politician or citizen to vote for civil unions is to ask them to give public recognition to acts which the church has always taught are, objectively speaking, gravely sinful, since they constitute a misuse of our God-given sexuality," he said.
He acknowledged "there is some justification to giving lasting friendships (whatever their nature) some legal entitlements similar to those of the next of kin".
"But these are rights that can be otherwise regulated by legal contracts. Redefining marriage is too high a price to pay," Fr Twomey said.
Writing in the February edition of the Word magazine, he said: "Ever since the 19th century, free thinkers have been trying to convince the public that marriage is simply a cultural artefact, something man-made and so to be changed according to felt needs. Their secular view was offered in order to redefine marriage. To a huge extent, it has succeeded."