Of the arguments against broadening the definition of marriage, Tony Abbott has found the most circular.

Marriage, he told his friendly radio facilitator Ray Hadley, has "stood the test of time".

Except that it hasn't, which explains the current push to have it reflect social values as they have steadily evolved - values increasingly at odds with discriminations long deemed acceptable.

Inadvertently, Abbott, who complains we are being "frog-marched" into change, had strayed into the realm of the reductio ad absurdum in which the fatal weakness at the heart of an argument is laid bare per force of its assertion.