Malcolm Turnbull has accused those who claim legalising gay marriage would undermine the institution of ''dripping with the worst sort of hypocrisy'', in a speech defying the Coalition's opposition to changing the law.

Mr Turnbull, a frontbencher who cannot vote for gay marriage because of Tony Abbott's refusal to give Liberals a conscience vote, said the ''deepest pools'' of this hypocrisy were all too often found ''among the most sanctimonious''.

''The threat to marriage is not the gays. It is a lack of loving commitment, whether it is found in the form of neglect, indifference, cruelty or adultery, to name just a few manifestations of the loveless desert in which too many marriages come to grief.''

Delivering the Michael Kirby Lecture in Sydney last night, Mr Turnbull said if Liberal frontbenchers had a free vote, he would support the private members' legislation voted on later this year.

He noted that ordinary Liberal MPs were not expelled if they crossed the floor, ''so in that sense every vote is a conscience vote''. But ''because the leadership are not permitting a free vote, shadow cabinet ministers are bound to vote in accordance with the collective decision''. If they wanted to cross the floor they would have to resign from the shadow ministry ''and I do not propose to do that''.

But Mr Turnbull offered a swingeing demolition of the case against gay marriage - and also pointedly said previously Liberals had been given a conscience vote on marriage issues.

''I am utterly unpersuaded by the proposition that my marriage to Lucy, or indeed any marriage, is undermined by two gay men or two lesbians setting up house down the road - whether it is called a marriage or not,'' he said. ''Do the bishops seriously imagine that legalising gay marriage will result in thousands of parties to heterosexual marriages suddenly deciding to get divorced so they can marry a person of the same sex?

''Are not the gays who seek the right to marry, to formalise their commitment to each other, holding up a mirror to heterosexuals who are marrying less frequently and divorcing more often?''

Mr Abbott has said marriage is between a man and a woman not just to fulfil their own personal happiness ''but because we have obligations to the children that come with families''.

But criticising those who objected to gay marriage on the grounds that children deserved to have a mother and a father, Mr Turnbull said that ''in an ideal world, as opposed to this vale of tears, the best parents for any child are their biological parents. However, in many cases one or even both biological parents are simply not there. And … not infrequently, even when they are there, one or both of the biological parents are neither loving nor wise.

''So the proposition that the ideal parents for any child are its biological parents is a statement with which we can all agree in the generality, but which does not apply, for one reason or another, in many particular circumstances''.

As for the argument gay married couples would be more likely to adopt children: gays were able to adopt in several states; adoption was a state issue, and many married couples today were not accepted as suitable to adopt a child.

Invoking the name of John Howard, Mr Turnbull said Mr Howard ''was not thinking of gay couples when he said in 1995: 'A stable functioning family provides the best welfare support system yet devised'. But the point is well made.''

With the legislation - there are separate bills from Labor backbencher Stephen Jones and Greens Adam Bandt - doomed to failure, Mr Turnbull said that even if the Liberals had a free vote, he thought the numbers would not be there to pass gay marriage. But they were certainly there for civil unions and ''we should not miss the opportunity to legislate for civil unions for same-sex couples in this Parliament''.