Every menstrual period of the female and every nocturnal emission of semen by the male are merely expressions of Nature's desire that the species shall be propagated. Nor is the perpetuation of the species left to a mere chance, as among the wind and water fertilized plants, in which instances the agencies of the elements are depended upon to disseminate the pollen; but it is rendered certain by the all conquering force of a natural instinct secondary only to the primal law of self preservation. This overpowering impulse is the sexual instinct, or sexual love. Thus Nature employs the force of the love-compelling sexual instinct to constrain males and females to mate in order to ensure a fulfilment of her designs, and not for a moment does she leave the future of the race to the caprice or whims of fickle mankind. Thus sexuality is at the bottom of all true conjugal love; and where there is no sexual attraction between individuals of opposite sexes, there can be nothing but a platonic love, which, being free from sexual longings, is merely friendship.
Among animals the manifestation of this law works along normal lines, and instances of perversions in which mere sensuality is practised are exceedingly exceptional among them. Thus Letourneau1 mentions as exceptions of gross ness that "the stupid tatoways [armadillos] meet by chance, smell each other, copulate and separate with the greatest indifference. Our domestic dog himself, although so civilized and affectionate, is generally as gross in his amours as the tatoway". In the same way some men, "although so civilized and affectionate", easily purchase animal love from prostitutes; but between this and a noble monogamic love there is the widest possible difference.
This normal sexual instinct, then, actuates men and women to love each other, to pair off in marriage, to found homes, and to provide for the expected offspring; and sexual feelings exercise a directive power over most of the activities of life moulding our religion, our literature, our art, our etiquette, and, in short, influencing almost every impulse of human endeavor which is not attributable to self preservation. "Were man to be robbed of the instinct of procreation and all that arises from it mentally, nearly all poetry and, perhaps, the entire moral sense, as well, would be torn from his life" (Maudsley).
The fulfilment of the promptings of sexual love, being a law of Nature, is a pure and chaste communion when it has in view the perpetuation of the species, and when it is exercised in the married state; but the lascivious gratification of sensual desire, which transgresses natural laws and actually aims to violate them, is a marked perversion which places those who indulge in it in a class by themselves below the level of the brutes.
Rightfully guided, this generative passion leads to the fullest enjoyments of life, such as home, wife and children, and to morality and every virtue. It is our most priceless heritage when experienced in its pure and natural glow, but is not to be trifled with without incurring the most inexorable punishments, notably the pollution of the very fountain-source of love. Among human beings, the simple physical enjoyment of the act of intercourse without a mental state at least akin to love cannot be anything but a disappointment; for it is wholly impossible to divorce the psychical from the physical sensations in this relationship. Therefore, as before pointed out, lewd men, whose brains have retained all kinds of corrupt impressions, often cannot enjoy the sexual act without substituting some sham in their imaginations which cheats them into the belief that they entertain feelings akin to love for their mistresses.
But love can be genuine only when the memory is not tinctured with corrupt impressions, and when the man and woman long to possess each other, body and soul, in the relationship of husband and wife. Such love brings tranquillity and joy dependent on a realization that the creative act is the highest function of manhood and womanhood, and on a full confidence that there is no moral or physical sin in the act; for pledges are given that they shall be mutually responsible for the results of intercourse. In man the longing for sexual intercourse is naturally more powerful than it is in woman, so that his role is that of aggressiveness in courtship; but on the other hand, sex dominates a woman far more, and by far the greater proportion of the reproduction and early nurture of the race is laid on her, so that she may well be deemed the most exalted of all created beings by reason of her physical nature.