Liberal Senator Cory Bernardi has suggested legalising same sex marriage would lead to demands for bestiality to also be sanctioned.

Speaking in Parliament last night on a gay marriage bill sponsored by four Labor senators, Senator Bernardi said the push for same-sex marriage was part of an attack on ''our enduring institutions.''

''It is another chip in the fabric of our social mores,'' he said. ''The time has come to ask, when will it end? If we are prepared to redefine marriage … what is the next step?''

''The next step … is having three people that love each other should be able to enter into a permanent union endorsed by society, or four people. There are even some creepy people out there, who say that it's OK to have consensual sexual relations between humans and animals. Will that be a future step?''

Labor Senator Helen Polley also spoke against the bill. She said two-thirds of the correspondence she had received on the issue had urged her to oppose change. She read aloud a letter from a constituent which said same-sex marriage could ''create another stolen generation by putting adult desires above the needs of children''.

But another Labor senator, Doug Cameron, said arguments about children being disadvantaged by being brought up be same-sex couples ''denies the reality of some children facing absolutely terrible lives with heterosexual couples''.

Labor Senator John Faulkner said the debate was not really about the value of marriage, or its role in society. ''It is a debate on the simple question of whether it is right for a government to deny some of its citizens access to a secular, government-recognised status on the basis of the gender of the person they choose to share their life with,'' he said.

Liberal senator Sue Boyce will defy Tony Abbott by speaking in favour of a bill to legalise gay marriage, and has not ruled out crossing the floor to vote for it.

Senators last night resumed debating the bill, sponsored by Labor senators Trish Crossin, Carol Brown, Louise Pratt and Gavin Marshall.

A vote on the private member's bill from Labor MP Stephen Jones, which is being debated today by the House of Representatives, is likely today or tomorrow.

A vote in both houses this week - which will see both bills defeated - would clear the issue off the government's agenda.

Senator Boyce, of Queensland, has previously publicly expressed support for same-sex marriage. But the Coalition has maintained a position of opposing change to marriage laws, and Coalition MHRs and senators are bound to vote against it.

''I support the intention of the bill,'' Senator Boyce told The Age yesterday.

Senator Boyce said she was still considering how she would vote.

Bestiality Enters Gay Wed Debate