This is basically an end around the First Amendment. While the government is not allowed to ban speech, so it pressures corporations to ban speech on its behalf, with an implied threat they will will hurt the corporation in some way if it does not comply.
Which is not actually legal. It's been found repeatedly in court that the government can't work hand in glove with a third party to violate civil rights. Otherwise the FBI would work hand in glove with third parties to beat people with rubber hoses until they confess.
Perhaps so, but the legality of an action is immaterial if it is left unprosecuted.
And I recently ran into a tidbit about the rules in Australia: Detailed pictures of female genitalia are automatically 18+, even if the purpose is educational in showing girls the range of normal genitalia to show them that theirs is normal.
Do you have a source or more information on this? I'm sure it was implicit due to social norms in decades past but I couldn't find anything recent about this online.