Senator Gary Humphries is warning that future legislation allowing same-sex marriage could be struck down by the High Court, leaving gay couples devastated.
The ACT Liberal says the Gillard government should not act ''irresponsibly'' by allowing gay marriage to become law.
''I think it is particularly dangerous to be proceeding on constitutionally shaky grounds,'' he said last night.
''Somebody who has been married in the belief that they are legally entering into an enforceable arrangement only to find a year later or whatever, the High Court strikes down that situation, is an incredibly difficult position.
''It seems to me that the Commonwealth should go to every possible length to avoid that occurring.
''It's utterly irresponsible to potentially put people in that position.''
Senator Humphries is deputy chairman of the committee which has released its report on the private members bill of Senator Sarah Hanson-Young, of the Greens, on the issue.
He is personally opposed to gay marriage but another Liberal on the committee, Sue Boyce, backed the majority report in favour.
The committee received an unprecedented 79,200 submissions, 46,000 of which were in support of same-sex marriage.
The six-member committee voted 4-2 in support of recommending changes to the Marriage Act to accommodate same-sex marriages and that the bill pass with minor changes.
Last week, a cross-party lower house committee inquiry into two private members bills on same-sex marriage tabled a report but declined to support or reject the legislation proposed by Labor backbencher Stephen Jones and Greens MP Adam Bandt.
Prime Minister Julia Gillard is opposed to legalising same-sex marriage and will not move a government motion to allow debate on the issue. But Labor MPs are allowed a conscience vote on the private members' bills.
Section 51 (xxi) of the Constitution gives the federal government power to make laws over marriage.
Senator Humphries said proponents of same-sex marriage should push for a referendum.
''They assert that there is strong support for same-sex marriage, then they would have little doubt they would get change through a referendum, but in the absence of that, I think it's very unsafe to proceed,'' he said.
''The committee heard strong legal argument that you can't assume the power over marriage assumes a power over other relationships we might like to call into the definition of marriage.
''When the Constitution was put together, marriage was assumed to be between a man and a woman.
''Suddenly that question is in doubt, but that doesn't alter the fact that that's a strong traditional understanding of what the word marriage means.
''To now argue that it may mean something else and we can use the power for that defined purpose for other purposes is I think quite inappropriate.
''It's either one thing or it's not.
''The Commonwealth can't acquire a power over schools by defining the section of the Constitution to give it power over 'lighthouses' to mean 'schools'.''