In endnote [A] to An Enquiry Concerning Human Understanding Hume asks: “I should desire to know, what can be meant by asserting, that self-love, or resentment of injuries, or the passion between the sexes is not innate?” He clearly thinks that these “ideas” (Locke’s term) are innate, so that they are not learned by experience or instruction. In particular, sexual passion is present innately, though it may not reveal itself until well after birth. We should not suppose that Hume means only some kind of non-cognitive drive for sex: he must mean that the affective-cognitive package of sexual passion is innate, including certain kinds of knowledge. Sexual passion includes the desire to do various specific bodily things, so it will involve some kind of mental representation of these things. It will involve knowledge of what to do sexually—what goes where, and so on. Thus some conception of the anatomy of the object of this passion will be installed in the mind prior to all experience. In short: sexual knowledge is innate.
Presumably this thesis will not be disputed in the case of animals. It is hardly to be supposed that dogs and cats, to take only the animals most familiar to us, learn how to have sex by means of observation or instruction: they are not in the dark about the mechanics of sex until they see other animals doing it or receive a “sex talk” from their elders. No, they know instinctively what to do—which is just what you would think the genes would program. Just as cats know instinctively how to fight and groom, so they instinctively know how to have sex. And not only the sexual act itself but also what leads up to it and what may follow it—they also know innately how to court and parent. This knowledge lies dormant till maturity, but it is encoded in the genes nevertheless. It is as instinctive as eating and breathing, walking and sleeping. Animals must have an innately fixed mental model of the opposite sex’s anatomy along with an action plan to guide their behavior. This innate system will be species-specific: cats and dogs don’t have mental representations of the anatomy of elephants or reptiles. They have a dedicated module for sexual activity equipped with suitable cognitive structures. Compare the human language faculty: a species-specific modular mental structure present at birth. This is just what you would expect given the importance of sexual know-how right off the bat (what would happen, or not happen, if an aroused animal had never observed an act of intercourse and had no innate knowledge of what it involves?).
Plato, Descartes, Leibniz, and Chomsky didn’t consider sexual knowledge, preferring to talk about knowledge of mathematics or grammar, but it is clear that they would agree with sexual nativism.  Poverty of the stimulus applies here, as well as uniformity within a species, brevity of the learning period, spontaneous emergence, etc. Also there is a complexity to sexual knowledge that is easy to miss: it is not just a simple matter of stimulus and response. When animals mate they have to negotiate each other in all sorts of ways, conducting an elaborate series of actions leading to a specific goal, preceded by appropriate courtship (think bower birds). The actions involved are not simply elicited by impinging stimuli but are carefully controlled by perception of the prevailing circumstances, involving knowledge of fertility and the correct way to mount and be mounted, as well as what to do once coitus is complete. As parenting is a highly structured activity, so is copulating. Neither is simply the mechanical product of randomly impinging stimuli. I am sure that if animals were artificially prevented from observing the sexual behavior of other animals they would still know how to have intercourse when the time is ripe. So we should add sexual knowledge to the list of other things now generally considered innate; and perhaps it could serve as the basic model for the nativist position, given its obviousness (compared to ethical knowledge, say).
Returning to humans, picture Adam and Eve: they have never heard of sexual intercourse and certainly never observed it in others. No one has ever taught them about the birds and bees. According to the empiricist, they know nothing of sex and are clueless about how it should be performed—as they are ignorant of tennis and how it should be performed. According to the nativist, they are already equipped with sexual knowledge (God would not leave them sexually ignorant, given the importance of reproduction for the human race). They innately desire sex and they innately know what to do to satisfy that desire—no need to experiment with various bodily entanglements in a process of trial and error. Genital pleasure will certainly offer them some guidance, but it will not by itself be sufficient to explain their sexual behavior, since they need to know how the genitals are combined in sex. There will necessarily be a cognitive dimension to this, presumably involving mental models of the other’s body and which parts do what. God will equip them with this knowledge, so as to prevent much blundering and potential injury. Adam and Eve were created already in possession of sexual know-how (which involves a good deal of know-that). Let us then suppose that they possess a specific schema or module or data structure dedicated to sex located somewhere in their brain, alongside similar centers for language, mathematics, ethics, and whatever else you deem innate. They possess an innate sexual faculty alongside other innate mental faculties.
Now my question is this: how extensive is this faculty? How much sexual knowledge is innate? Is there knowledge of the various possible sexual positions or the various kinds of foreplay? I doubt it, but that is not my chief concern, which is this: do humans have innate knowledge of the connection between sex and reproduction? Do they know instinctively that sex produces babies? Certainly the sentence “Sex causes babies” is not analytic—there is no way to reason from propositions about sex to propositions about babies. It is entirely possible to be ignorant about the origin of babies while being well informed about the mechanics of sex. It is an a posteriori empirical fact that sex leads to babies. I suggest that people don’t innately know that sex and reproduction are connected; they learn this from experience, just as the empiricists say. And they may not learn it—they may remain ignorant of the connection. Consider animals again: I conjecture that no animal has ever grasped the sex-baby connection. Animals innately know how to have sex and how to take care of babies, but they don’t know—innately or by experience—that sex results in babies. To know that requires fairly sophisticated empirical enquiry concerning the time period of pregnancy as well as acceptance of something quite surprising. I suspect that even our most intelligent cousins in the animal world find the origin of children a mystery, if they think about it at all. Babies just appear from nowhere as far as they are concerned (how on earth did they get inside the mother?), even though they have knowledge of the activity that in fact produces them. (Indeed, I doubt that animals ever wantoffspring at all, though they clearly do want sex. ) When did it dawn on our ancestors that sex produces babies? Did Neanderthals know about the sex-baby connection? Are there any “primitive tribes” today that don’t grasp the connection (thinking that babies are implanted by the gods and that sex is just good fun lacking any causal connection to reproduction)? This is why it is possible to convince children that the stork brings babies—because they don’t know otherwise (try convincing anyone that sex is performed with the ear and the little finger and see how far you get!). So some sexual knowledge is innate and some is acquired. Well and good: it is the same with knowledge of language–some is innate and some acquired. It is an empirical question where a given piece of sexual knowledge falls; there is no logical necessity here.
But now there is a puzzle: why is this kind of sexual knowledge not innate? You would think that the genes, concerned above all with promoting reproduction, would program knowledge that sex leads to children, so that animals would do what is necessary to create children (new bodily vehicles for genes to nestle in). Their practical reasoning would take the form: I want babies; sex leads to babies; therefore I want sex. But this is not how it works (notoriously and tragically): animals and people want sex and as it happens sex leads to babies. Animals don’t have sex in order to produce babies; they have sex in order to have sex. But why didn’t the genes at least let us know that sex produces babies? They gave us other kinds of sexual knowledge, but they kept us in ignorance of this central fact about sex. We know innately that the penis and vagina are involved in sex, but we don’t know that children are—unless by experience. Why don’t at least some animals know this? It’s almost as if the genes were keeping this piece of information from us! They don’t want us to know that sex leads to babies, but they very much want us to know how to have successful sex (by “us” I mean all animals).
Here an obvious thought comes to mind: the genes don’t want us to know because it might deter us from doing what they want us to do. If animals were well aware that having sex would lead to becoming parents, they might be less inclined to have sex. That is surely true in the human case, but from a gene-theoretic perspective it makes sense for all reproducing animals: animals are the slave of their genes and their genes keep them in ignorance of what might lessen the genes’ chances of survival. So maybe the ignorance is programmed ignorance—ignorance by design.  The genes have made sex very attractive for animals (I’m thinking mainly of mammals) so that animals will do what the genes want them to, without letting them in on the dirty little secret that sex leads to parenting. We can imagine a species in which the psychology is constructed differently: the animals want to have children but the sex is not particularly appealing, rather like building a nest or burrowing a hole. The genes have designed these animals in such a way that their desires lead to offspring and hence gene propagation, but without making sex attractive and the sex-baby connection obscure. Theoretically this would work to get the genes into later generations, but evidently it has not gone that way on planet earth; the genes here deemed it wiser to opt for attractive sex (along with knowledge of its mechanics) combined with ignorance of its effects. Boring sex might in principle lead to babies in the presence of a desire to have babies, but the genes have in their wisdom opted for enjoyable sex and a lack of knowledge of the sex-baby connection. We must assume they had their reasons. The psychology of sex is therefore not one in which the sex-baby connection plays a role, however important it is biologically: animals don’t have sex as a means of satisfying their desire to have babies; they have sex despite the fact that it leads to babies. At any rate, they operate in ignorance of the fact that sex produces babies. Humans are probably alone in the animal world in grasping this connection, as we are alone in knowing many things of no biological relevance; from a biological point of view, this knowledge is either unnecessary or positively disadvantageous. Genes manipulate their vehicles by means of orgasm and by not letting them know that orgasm leads to parenting. Hence animals have innate sexual knowledge but not innate knowledge of the results of sex: they know the mechanics but not the consequences. There is really no reason for any animal species to grasp the fact that sex leads to babies—the genes can get along quite nicely without ever installing such knowledge, as they did for millions of years before modern humans came along. Thus our sexual knowledge falls into two parts: a necessary part (the mechanics) and an unnecessary part (the consequences)—the former being innate and the latter acquired.
Socrates elicited knowledge of Pythagoras’ theorem from the slave boy by suitable questioning, thus demonstrating his innate knowledge of geometry. I doubt that questioning alone could elicit the slave boy’s innate knowledge of sex, but the injection of a suitable hormone might have done the job, as it naturally does in the case of adolescence.  However, nothing like this could elicit the knowledge that sex produces babies—for that the slave boy would need either instruction or experience. So nativism about sexual knowledge is partly right and partly wrong: some of it is inborn and some of it is derived from experience. We are certainly not born a sexual blank slate, as Hume observed.
 They may have thought it just too obvious to be worth mentioning compared to knowledge of language and mathematics; or perhaps they had a non-cognitive view of the sex instinct. Hume is bringing some solid common sense to the discussions of innateness with which he was familiar, notably Locke’s. Sexual passion is clearly not the result of being instructed to have it, or observing others in flagrante!
 I mean that they don’t formulate the thought that it would be nice to have children and then set about accomplishing that goal—the thought of children probably never enters their heads as they copulate.
 We might compare this with the ignorance of death among animals. They don’t know they will die, either because there is no biological point in knowing this or because the genes have expressly ensured such ignorance (a mutation that produced knowledge of death might make animals with it less reproductively successful than animals without it). Or again, how would it benefit the genes for animals to know that defecation is the result of eating? Animals probably have no idea about this causal connection—and why should they given that it isn’t useful knowledge (unlike knowledge of how to copulate, or fertilize eggs in some other way)?
 I assume this is the mechanism, simply stated: it is some change in the nervous system that brings the implicit innate knowledge to the fore upon sexual maturity, just as such changes produce sexual maturity itself.