Rape is not simply about sex, it is about power over the individual, anger issues, and so on...
It has little to do with civilisation and what your Science teacher failed to get across to you is that even in primitive and pre-civilisation societies sex was generally consensual, so while it may have been polygamous, brutal and seemingly random it was in fact a cumulation of a complex and specific courtship, not unlike any other animal we see today.
All civilisation has done is added to the evolution of that courtship and it has become more complex and advanced, although when you strip away all that sex is still fundamentally about procreation and the evolutionary pressure that implies on the individual. Some are able to control and/or express that mutually and some are unfortunately not able to do so.