Same-sex marriage should be legalised for the "common good", one of Australia's leading Catholic thinkers has urged, arguing civil marriage cannot be seen as an instrument of the church.

In a marked departure from other Catholic leaders and many Coalition conservatives, Jesuit priest Frank Brennan said any concerns about religious freedom should be set aside until after a successful "yes" vote in the postal survey.

And he turned opponents' fears on their head, saying the increasing prevalence of same-sex couples with children was an argument in favour of marriage equality, not an argument against.

"We've got to factor that in to the common good argument about what's necessary," Father Brennan told Sky News on Friday, following his delivery of the Lionel Bowen Lecture this week in which he declared he would vote "yes".