A proposal to allow a free vote on the Civil Partnerships Bill in the Seanad has been defeated by 44 votes to six today.
It means that the three Fianna Fáil senators threatening to oppose the Bill will have to defy the party whip if they want to do so.
Senators John Hanafin, Labhrás O'Murchu and Jim Walshe are refusing to support the Bill, with Senator Walshe having confirmed his intention to vote against it.
Senator Walshe says while he can accept diversity in society, he cannot accept anything that damages the institution of marriage.
"Existing marriage rights should remain unique to marriage, because of the uniquely pro-child aspects of marriage," Senator Walshe said.
"It is not discrimination to treat a unique institution - like the marriage of a man and a woman - in a unique manner."
The legislation gives legal rights to gay couples for the first time.
However the only openly gay member of the Oireachtas - Senator David Norris - also said he won't support the Bill because it doesn't properly protect the rights of children.