Supports of gay marriage oppose it because it's a foregone conclusion (most polls find a solid majority in favor of gay marriage), a hugely expensive exercise and non-binding to boot.

Opponents of gay marriage oppose it because having rock solid data about the level of support would make "but won't somebody please think of the children!" type fearmongering harder.

TBH, I'm not really sure why anybody even in government supports it, but that's Aussie politics for you.