Specifically it's the "implied sexual violence" that they find objectionable, which they rule to be "not justified by context." The player is also rewarded or incentivised to use "illicit or proscribed" drugs.
It's like they encourage you to eat mushrooms and grow big or something. Just because they've got an R18+ doesn't mean they still don't hide under the skirt of MA15+.
"In the Board’s opinion, Saints Row IV, includes interactive, visual depictions of implied sexual violence which are not justified by context. In addition, the game includes elements of illicit or proscribed drug use related to incentives or rewards. Such depictions are prohibited by the computer games guidelines," said acting director Donald McDonald.
First off - if your family name is McDonald you don't call your boy Donald - second, the rest of the world is pretty happy to let Saints Row IV peddle its toilet humour, so why is the Australian Classification Board being square about it?
According to their guidelines, it is ineligible for classification if it would "depict, express or otherwise deal with matters of sex, drug misuse or addiction, crime, cruelty, violence or revolting or abhorrent phenomena in such a way that they offend against the standards of morality, decency and propriety generally accepted by reasonable adults to the extent that they should not be classified.” In other words, anything to make the moral brigade drop their monocle.
Saints Row IV releases on Xbox 360, PS3 and PC August 20th in the US, 23rd in EU.