Rape..... First of, couldn't get or keep an erection if I was to perform rape, but I would'nt confine rape solely as an act of mentally disturbed people.
Look at the way cats have sex.... The male cat have to wrestle the pussy that will respond fiercefully with biting and claws until he can grasp the pussy in her neck slin, which will make her surrender.
This is a common reproduction ceromony amongst felines.
I would call it "functional rape" as the male have to "prove" to be a strong and agile enough to be worthy to helpt the pussy produve offspring.... Obviously this is kind of negotiated rape.
About Humans, in more , what we call "primitive" tribal cultures we see some of that functional semi-negotioated rape being integral part of that culture (i.e. "stealing wimen from neighbouring tribes for fresh DNA) , but in less harsh circumstances, in a well fed modern soceity, rape has no place in our system and rapes are believed to be commited by sickos and dropouts... or?
NON-NEGOTIATED RAPE
Only as long as we can maintain peacefull and prosperous economical/political circumstances we can convinve ourselves that we are "truly" civilized people, What happens if war breaks out?
Typically we see that the nice guy next door (not your average sicko) and his platoon will ransack an "enemy" village and rape the females (just look at yugoslavia, a formerly "modern western soceity"). This guy wouldn't do such an act on his own, but males in packs become wolves and most of them rather follow a "leader" than rely on their own conciousness (the ones who do, are or outcast or leaders). Also keep in mind that these guys are in a situation of "live or dead" an enemy bullet with their name on it could end the game any minute, combined with the increased testosterone response of going to battle I think it could be enough to send a relatively sane and normally nice guy on a desperado rampage.....
EXERCISING THE BEAST
Even if enough money/food around modern people feel like they have to satisfy the animal parts of the brains hence the negotiated S&M games which have the unique aspect for females that they can be dominant eventhough their slaves might be stronger psycally than them (ok i guess that Nicole Bass COULD overpower some guys and i believe she is in the porn bussiness lately doing just that)
People explore the boundaries of the acceptable because it is exciting and you feel what it's like to be alive when you risk dead (look at extremesports like basejumping, wouldnt get that adrenaline rush if it was 100 % safe!)
Personally I cannot imagine finding satisfaction or need for classical rape, but I honestly cannot tell you what I would do in times of heavy war...!
I do like to "get rough" sopmetimes on a girl that is kinda "asking for it", especially on a warm midsummer night but ultimately this is negotiated jungle fever between two modern humans that both like to exercise the beast....