Illustration: Ron Tandberg In what could be construed as encouraging backbenchers to do so, 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''. Mr Turnbull offered a swingeing demolition of the case against gay marriage - and also pointedly said that 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?'' he said. ''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 neitherloving 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 concerns that gay married couples would be more likely to be able to adopt children, he said 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 there was a strong public interest in people living together and supporting and helping each other. 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.'' Codependency was economically good for the state, Mr Turnbull said. ''There will plainly be less demand for social services, medical expenses, hospital care if people, especially older people, like Michael [Kirby] and [partner] Johan, live together as opposed to being in lonely isolation consoled only by their respective cats''. Loading

With the legislation - there are separate bills from a 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''. Follow the National Times on Twitter: @NationalTimesAU