Dolphins use sex as an integral part of their complex social webs. Male bonding helps them form teams of three to six dolphins. These teams then work to kidnap females and fend off other groups of rivals. Many like to point to this scheme as most like our own. There is even a case to be made for rape as the natural model for humans.
True rape is rare in nature.
In most cases, a male has no interest if the female is not in estrus. What's more, in most species the females will block the male if they're not ready. But the male orang-utan can expect a fight anytime he wants sex. This may be because the orang-utans have sex face to face like we do. This means he must approach from the front.