What rape is not a biologically deviated behaviour depends on whether we decide to accept the following assumption about human nature or not: The main general features of our species (language, moral sense, intelligence, personality, attractiveness between the sexes) are genetically programmed. The environment contributes to the flowering of these innate abilities, but it works upon material already structured. The mind is equipped with emotions, drives, and faculties of reasoning and communicating -shaped by natural selection- that act over the course of human evolution, and owe their basic design to information in the genome. (1)
The notion of an innate human nature does not threaten standard liberal values. For example, political equality is not a factual claim about the distribution of inbred abilities. It is a prescription, intending to give people with the same abilities equal opportunities. Hence, evolution is a ruthlessly amoral process, often resulting in a brutish and long one. (4)
There are many aspects of human nature -violence, selfishness, ethnocentrism, prejudices, lying, and cheating- that have an evolutionary basis. The moral sense is the part of our nature that passes judgment on these natural foibles, and it is a powerful force in human affairs. In such a way, rape may be biologically natural, while being immoral, because it is an effective way to get genes into the next generation if there are no other options. (3)
Chimpanzee female doesn't like to mate her maternal brothers. Even when those males court elaborately, with shaking branches and rude stares and proud postures, female chimpanzees refuse their brothers. Normally, the female's reluctance to mate with her brother marks the end of it. But occasionally a brother can't stand being denied. She resists and avoids him. He becomes enraged. He chases and, using his greater size and strength, beats her. She screams and then rushes away and hides. He finds her and attacks again. He pounds and hits and holds her down, and the rape consummates. (2,5)
Sharing with chimpanzees the propensity to ravishing make humans a little less special: no species is more closely related to humans than chimpanzees are (DNA divergence between both is less than 5%.) These species separated 5 million years ago, and because of chimps remained in the not much changed rain-forest environment since then, they are proper models of our common ancestors.
On the above basis, should we suppose that genes make certain actions inevitable? The answer is 'no'. Human genome may make rape more probable than it would be without its existence, but it certainly does not determine a person to rape. For rape to occur, the countervailing forces of conscience and prudence have to fail to prevent what is only a latent tendency.
References
1. Matt Ridley. The Origins of Virtue.
2. Richard Wrangham, Dale Peterson. Demonic males: Apes and the origins of human violence.
3. Steven Pinker. How the mind works.
4. Steven Pinker. The blank slate: the modern denial of human nature.
5. Hate and evolutionary psychology