The Constitutional Convention has heard that legalising same-sex marriage would improve the status of gay and lesbian couples in Ireland in over 160 ways.
However, the Catholic bishops told the Convention that the move would make the roles of mothers and fathers completely irrelevant in the bringing up of children.
Thirty-three seats at the Convention are allotted to TDs and Senators.
The remaining 66 are drawn from a representative cross-section of the Republic's adult population.
Fine Gael has not endorsed legalising same-sex marriage, but the other main political parties have, while voters are divided on the issue.
Just over 1,000 couples have entered Civil Partnerships in Ireland over the past two years.
Gráinne Healy of the Marriage Equality Campaign told the Convention that Civil Partnership is inferior to same-sex marriage in over 160 ways.
She said one example is that that gay and lesbian couples are prohibited from adopting a partner's biological child who they are raising together.
Bishop of Kilmore Leo O'Reilly told members they were being asked to consider if, for the first generation in human history, the roles of the mother and father are to be consigned to history.
Following another set of round-table discussions and a panel discussion on the issue the participants will vote tomorrow on whether to accommodate same sex marriage in the Constitution.