So, it appears that ‘sex in a motel room on a businees trip’ is not a work related activity. And injuries because of it also not claimable under any worker compensation scheme. So says the High Court in Comcare v PVYW  HCA 41 after other courts held the opposite.
It appears that for the HC the main consideration is whether the employer induces or encourages the particular activity resulting in the injury. Of course, no employer would ever encourage an employee to have sex while away from the office (at least not openly). The decision is positive for employers as there now wont be a need to spell it out, i.e. provide detailed policies on what is allowed or not allowed. Common sense, really, although it remains unclear what can be safely assumed to be ‘encouraged’ and what is not.