In light of recent developments, I think it's important to decide what rating we are going to comply with and stick to it no matter what. The options are following (mixed for both US and other international standards):
12 (it exists for movies in the US and AFAIK also for games in some European countries)
13 (ESRB Teen)
15 (it exists for movies in the US and AFAIK also for games in some European countries)
16 (not sure whether it still exists, I think in some European countries it did/does exist)
17 (ESRB Mature)
18 (ESRM Adults Only)
Pls leave feedback, not just a vote.
Explicit sexual references require AO (18), whereas if there are no such references Mature (17) always suffices, even if there are extremely violent and bloody things, like skinning people alive, genocide, wholesale slaughter, and the like (OTOH if you add to such a brutal stuff a single nude lady's picture, it's instantly AO). Not 100% sure what the two bare-breasted girls in the stained glass paintings in one of the UT maps converted to KA mean for us, dunno whether UT was Teen or Mature.
The original Hexen had a Teen rating AFAIK, meaning that kind of graphical violence is allowed for 13-year-olds.