Because society has randomly decided, just like all of its laws, that rape is bad. Sure, you're allowed to lock people in cages for the rest of their lives, execute them, force them into schools and brainwash them into lifeless, self-loathing, suicidal nutcases... BUT WHATEVER YOU DO, DON'T STICK YOUR PENIS IN THEIR VAGINAS! THAT'S THE WORST CRIME THERE IS!!!