Human psychology. Partially cultural, partially genetic, where some is instinct and some personality. The impact of rape also very much depends on the situation. Being intimately violated against one's will causes all kinds of partially persistent negative emotions that can have a profound effect on the level of anxiety in perceived risk-situations and all kinds of mental issues.
So it's not a matter of "What's the big deal, unless it gave you an injury or disease?" It's an issue of "How bad will the victim feel for how long and how much will that impair their life?" So you treat humans as mechanisms, as non-flawless software. Their software bits get corrupted when their hardware bits get violated, basically.