The reasoning behind the ban on same sex marriages was our previous Prime Minister, little Johnnie Howard, decided the Australian public did not want gay marrriage. He didn't consult the public, just decided he knew and that was that. Not just marriage either, but he blocked the ACT Government from making changes to their legislation that gave gay couples the right to enter a civil union.
The part I find the most ironic is the arguments against gay marriages usually come from those who also claim to believe in a minimalist government, with minimal involvement with the public.