You have to remember that very few animals have the kind of sexuality that humans do. In most species, the females have an estrus cycle. They are physically capable of having intercourse only during a short period of time (e.g., in dogs and cats it's about six days, twice a year), during which time they are fertile. During that time they exude pheromones and drive the males crazy. Stallions, bulls, and rams fight -- sometimes to the death -- over the privilege of having sex -- and of course their babies are born with the ability to run so can get by without much fathering. Dogs are more prolific, every male in the pack gets his turn with any female in heat so they all think they're the babies' father and will protect with their lives the little blind pups who can barely crawl.
When the females are not in estrus they exude no pheromones and the males don't even think about having sex with them. Couldn't get an erection to save their life.
Humans are practically unique. Our females are capable of having intercourse at any point during their fertility/infertility cycle, which happens once a month -- astoundingly frequent for an animal of our size with a gestation period of nearly a year. Our males don't need a burst of pheromones in the air in order to feel like having sex. We can couple anytime we want. And unfortunately, a male can force a woman to couple when she doesn't want to -- because they are both physically capable of doing it even if one is forced to.
One of the few other mammals that is not bound to an estrus cycle is the dolphin. And, of course, one of the few other mammals that is capable of enacting a rape is the dolphin.
I can't cite a reference -- although there's probably one on the web these days, everything else is. About fifteen years ago I read an article reporting that the dolphins off the coast of Australia engage in rape. In a three dimensional environment, for an animal with no arms, it's not easy. So they have to do a gang rape. Three bulls corner a cow and take turns, one having intercourse while the other two pen her in and keep her from swimming away. And three more participate -- they stand guard a short distance away to prevent anyone from coming to her rescue.
Six on one. Is that chickenshit or what?
Fortunately this is not universal behavior in dolphins. No one (at least at that time) has ever reported seeing this happen in other populations of dolphins.
At any rate, rape is physically impossible in most species. Unfortunately, in one of the most intelligent, charismatic species we know, it is not only possible but it happens.