That is an altogether different argument, namely that behavior (rape) is linked to physiological impulses (sexual arousal). Theory of evolution does not have to say much about individual behavior - it is concerned mainly with genetic behavioral traits that create reproductive advantage i.e. a greater chance to pass genetic traits to subsequent generations of offspring.
The fact of the matter is that mating behavior in humans is regulated by social institutions - it has been for at least 10 thousand or so years. While these institutions vary from society to society, they have one thing in common - they all are amied to prevent random sexual encounters and chanel reporductive behavior into legitimate relationships. In other words, their very purpose is to prevent rape, among other things. Therefore, from an evolutionary point of view, if rape is genetically motivated, as so-so biologists claim, then rapists have a fewer chances of passing this genetic trait on their offspring than non-rapists because of negative selection effect of social institutions.
That is altogether different from arguing that rape originates in biological instincts, as you claim, or for that matter, any kind of innate impulses. This is, in fact, a rehashed version of Freudism arguing that all human behavior originates in subconsciuus desires of pleasure and gratification. It it a concept of the human nature that has nothing to do with the theory of evolution (as science, not teleology and legitimation mythology). Evolution is about a change and specialization, whereas biological instincts and urges is something supposedly primordial, shared by all species, i.e. something that is universal and does not change very much.
Thornhill and Palmer's argument is shaky on still another ground - it tries to link a culturally defined behavior to genetic makeup. Rape is a meaningless category outside social institutions - it rests on a distinction between legitimate and illegitimate means of engaging in sexual acts. For example, family elders deciding whom their offspring should marry is not considered rape - even though it involves coercion to have a sexual intercourse against the parties' will or desires. On the other hand, consensual sex often involves varying degrees of physical coercion (e.g. BD). Rape can thus be meaningfully defined only as a violation of socially constructed sexual norms. From that point of view, rape is no different from any other transgression, such as sedition, illegitimate killing or taking someone else's property.
Arguing that propensity to violate social norms is biologically determined can take two forms: that either all members of a species are prone to a particular behavior because of their biology, or that some members are more prone because of differences in their biology. The first argument is probably true, but it is also trivial and inconsequential. Urination is not socially constructed, but biologically driven. Yet that does not explain the fact why some people violate social norms by urinating in public or, say, pissing on the Old Glory or Holy Cross to make a statement (in my college youth I pissed to holy water in a church for that reason). In the same vein, pursuit of a mate is biologically driven, but that does not explain why some people violate social norms in that pursuit. So if Thrornhill and Plamer argue just that - their argument is probably true, but trivial.
If, on the other hand, they argue that some individuals have a different biology than other that predisposes them more to violate social norms - as propping their arguments with references to evolution seems to suggest (because evolution is about specialized traits and behaviors not general ones) - they are rehashing old discredited theories of Lombroso who argued that criminality is in the genes, or for that matter, the Bell Curve crap. It is crap not because it challenges the liberal consensus (which is a good thing) - but because it is ex post facto rationalization rather than a causal argument, and thus a fallacy. That is, the authors attribute the purported but unobserved causes to the known behavior, and construe the lack of that behavior as the absence of the purpored cause (the fallacy known as denying the consequent).
The argument is fallacious because there is no way of proving it wrong. To prove it right or wrong, we would have to determine the purported cause i.e. identify the set of genes that is hypothesized to produce the behavior to be explained. The hypothesis would be confirmed if knowing whether individuals have or do not have that set of genes led to accurare predictions of their behavior (i.e. those having the set of genes violating social norms whereas those who do not have it - do not). It would not be confirmed if people with the hypothesized set of genes did not engage in transgressions any differently than those without that set.
Perhpas that test will be possible to carry out when we map all human genes and understand their exact functions and connections to physiology and behavior. But before that happens (and I understand it will not happen any time soon) - all arguments linking observed behavior to unobserved (purported) "biological" causes are sorcery not science.
wojtek