Liberal Senator Cory Bernardi has suggested legalising same sex marriage would lead to demands for bestiality and polyamory 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?"