Monosexuality and bisexuality (attraction to one and more than one gender/sex, respectively) are historical constructs, as are monomodal (e.g., gender/sex-based) and multimodal concepts of erotic attraction. I provide a brief outline of distinctions between single-gender and multi-gender attractions as they emerged in continental Europe. Nineteenth-century conceptualizations of sexual orientation in terms of gender-exclusivity were animated by medical frames for socio-sexual disfavor and aversion. From the early 1880s bisexuality became framed as a stage of "sexual inversion," and, from 1891, associated with notions of gender-independent attraction to particular "types." German and Dutch surveys reported in 1904 were pivotal in popularizing and internationalizing bisexual interest as a sexological intrigue.
Keywords: Bisexuality (historical perspectives); Eugen Wilhelm (1842–1923); medical history; monosexuality; nonmonosexuality; plurisexuality; psychosexual hermaphroditism.