Regarding the slaughter analogy: it's not explicitly about murder, but killing. Slaughter doesn't apply only to humans, of course. Slaughter is typically associated with the killing of animals for food, after all. The difference between rape, killing, and stealing, en mi opinion, is society still grapples with killing and thievery, for certain edge cases. There's no such gray area for rape.
Stealing if you're starving isn't condoned, but people do have more sympathy if you have extenuating circumstances. Murder is wrong, of course, but killing people in self defense is understandable. Killing animals for sustenance is okay. We need to eat or we'll die. Killing animals for pleasure is also okay. I could cut out steaks/Popeyes/breakfast sausages from my diet, but I've calculated the cost of a dead cow/chicken/pig is worth a nice meal.
There's no equivalent case in society where people find it defensible to rape.