Catholic bishops are to write to assembly members urging them to vote against a motion which would legalise same-sex marriage.
The Presbyterian Church has also written to politicians re-stating its opposition to any change in the legal definition of marriage.
The motion, tabled by Sinn Fein, is due to be debated at Stormont on Monday.
Fr Tim Bartlett said marriage between a man and a woman "still matters in society".
Fr Bartlett, from the Catholic Council for Social Affairs, was speaking on the BBC's The View programme on Thursday.
He said: "Any debate about this matter from a Christian point of view has to be framed in terms of respect, love, kindness towards each other and for each other as human beings and respect the dignity of each other.
"Mums and dads are important and that difference between men and women in the rearing of children is important and society has always recognised that and given it a special place."
On Monday, the assembly will debate a motion that calls on the Stormont executive to bring forward legislation that would allow for same-sex marriage.
In its letter, the Presbyterian Church has urged MLAs not to support the motion.
Last year, assembly members rejected a proposal that same-sex couples should have the right to marry.
Ahead of that debate last October, the Presbyterian church wrote to all assembly members stating its opposition to any change in the legal definition of marriage.
In its latest letter, signed by the co-convener of its church and society committee, Dr Norman Hamilton, it calls for "a much needed debate across all sections of our community on the role of the state in shaping a wide range of public and private relationships".
The church claims "renewed pressure" for a change in the law "suppresses and skews" that debate.