If insemination were the sole biological function of sex, it could be achieved far more economically in a few seconds of mounting and insertion. Indeed, the least social of mammals mate with scarcely more ceremony. The species that have evolved long-term bonds are also, by and large, the ones that rely on elaborate courtship rituals.... Love and sex do indeed go together.

On Human Nature (ed. Harvard University Press, 1978) - ISBN: 9780674016385