This.
I used to have a LARP (Vampire : the Masquerade) and quickly found it a good idea to give some guidance and boundaries to players about what to expect and how to act. We insisted to every single player about two sentences from the core rulebook:
Which was more of a disclaimer than anything because horrible things were going to happen in the story and we didn't want people who might easilly be offended mistakenly joining the game but more importantly we insisted on :
This means that you shouldn't revel in the barbaric stuff your character does, it has to put you ill at ease. Watching a good horror movie should put you ill at ease and so should playing a monster. Also if your character is smart, it's afraid too.
We backed this with the system as much as we could. For instance we asked the players the repentence for their cruel acts (even when it was necessary to their survival) manifested itself. If the character didn't feel bad for the action, there was no humanity roll, the loss was immediate. Because that's what humanity in V:tM represents. If you are familiar with that game, you know that it can make a character unplayable very quickly.
By insisting that horror is actually horrific, we had no rape incident at all (and if they raped an NPC, they would have lost humanity over it for no gain whatsoever too).
Plenty of horrific (non-rape) actions were done during the course of the two years I ran the game. Not one rape, and no one ever had to use the sentence "This is what my character would do."
Closest we have is a girl who made a female cop (NPC) addict to her blood (and therefore to her) because she needed a law enforcement contact and decided that even if the character didn't feel the need for sexuality (being undead and all) and wasn't gay during her life, she would have regular sex with her because otherwise she would feel bad about enslaving her and not paying attention to her. As a storyteller, I loved the twisted character logic.