Schopenhauer argues that love is (really) the individual human experience of a universal human impulse to procreate, and further that procreation should be (ideally) between a man and woman who are compliments of one another, in order to form a neutral product. The first function of love thus serves a philosophical or teleological anthropology, while the second function of love concerns a heteronormative ethics of procreation.
Schopenhauer’s teleological anthropology is shaped through a notion of love, as the “a very decided, clear, and yet complicated instinct - namely, for selection... of another individual, to satisfy his instinct of sex,” that functions as a bridge between two registers, the human individual and “something higher, that is, the species... as an immortal being is to a mortal... as infinite to finite” (5, 9). This is not to say that love is thus a peaceful bridge, however - love is “both the weal and woe of the species” (3). Or, love connects the species and the individual by pitting them against one another: “As a matter of fact, the genius of the species is at continual warfare with the guardian genius of individuals; it is its pursuer and enemy; it is always ready to relentlessly destroy personal happiness in order to carry out its ends; indeed, the welfare of whole nations has sometimes been sacrificed to its caprice” (13). As all is fair in war, love even resorts to deception, promoting a “secret task” wherein “Nature attains her ends by implanting in the individual a certain illusion by which something which is in reality advantageous to the species alone seems to be advantageous to himself... This illusion is instinct” (9, 4). In this sense, (and in a way that is inherited quite directly from Kant’s anthropology, see “On Education”), Schopenhauer develops a teleological anthropology in which individual humans have a drive to screw themselves over, e.g. committing themselves to another or an Other while under duress of illusion (“the illusion necessarily vanishes directly [once] the end of the species has been attained”), and “striving to perpetuate all this misery” (14, 15).
Schopenhauer’s ethics of procreation is heteronormative because it obliges procreation that is between a man and a woman (i.e. biologized procreation, “real aim is the child to be born”) and assumes that a neutral view or being can be attained, “the two persons must neutralize each other, like acid and alkali to a neutral salt... in order to complete the type of humanity in the new individual to be generated, to the constitution of which everything tends” (5, 8). It is not clear to me how Schopenhauer moves from his teleological claims about the human species (qua biologized universal) to his claims about the ethics of procreation (qua force of the universal), since human ideals have expressed themselves in human individuals by aiming at non-biological procreation, e.g. artistic or educational, and since the established teleological anthropology (i.e. humans are self-occluded and self-defeating) suggests that we are fundamentally lacking and so sexed, not whole nor neuter. It seems to me that the idealization of humanity as potentially complete and perfect is incompatible with Schopenhauer’s fairly pessimistic teleology, and this is why we see Schopenhauer’s simultaneous/inconsistent acceptance and rejection of humanity’s perfection, “the type of the species is to be preserved in as pure and perfect a form as possible... different phases of degeneration of the human form are the consequences of a thousand physical accidents and moral delinquencies; and yet the genuine type of the human form is, in all its parts, always restored” (5). Thus we can raise an objection to Schopenhauer’s claims - that “The particular degree of his manhood must exactly correspond to the degree of her womanhood in order to exactly balance the one-sidedness of each”; that two “may be so physically constituted, that, in order to restore the best possible type of the species, the one is the special and perfect complement of the other”; and that “This purpose [the secret task of the species] having brought them together [i.e. through an illusion of agency and happiness], they ought henceforth to try and make the best of things” - two different biases don’t make a neutrality, so under the given teleological anthropology we are better off dispensing with our fixation on successful neutrality in favor of a serious engagement with choosing the best way to fail or be partial (8, 9, 14).