Missionarius (a missione) est sodalis cuiusdam religionis qui ceteros religionis aliae convertere ad propriam temptaret. Usus missionariorum saepe conectitur Christianitati, sed vero et discipuli Buddhismi theoriam missionaricam invenerunt.
To be in line with history, this post should continue in Latin, the language most used for sexual matters not intended for everybody's ears. However, both our sources being English, we'll restrict ourselves to a simple horresco referens.
At the risk of disappointing you, the question treated here is a purely etymological one: where, when and how did the term missionary position originate?
|Missionary in vertical position|
Most persons will be surprised to learn that positions in intercourse are as much a product of human cultures as languages and clothing, and that the common English-American position is rare in some other cultures. (...) It will be recalled that Malinowski (1929) records the nearly universal use of a totally different position among the Trobrianders in the Southwestern Pacific; and that he notes that caricatures of the English-American position are performed around the communal campfires, to the great amusement of the natives who refer to the position as the "missionary position". (p.373)Now Malinowski's The Sexual Life of Savages in North-Western Melanasia (1929) can be consulted online. (Relevant pages here.) The word "missionary" occurs seven times, but not in the section The act of Sex (pp.335-343). We do find contempt of the natives for the "missionary fashion" of holding hands and leaning against each other, called "one of the novel immoralities introduced by Christianity" (p.479). Also, it is recorded that it is one of the special accomplishments of native cook-boys and servants (...) to imitate the copulatory methods of their masters, a certain Gomaya being perhaps the best actor in this respect. So it would appear that Kinsey concocted a condensed quote out of these several strands, adding campfire performances as couleur locale. The exotic information may have been Malinowski's, but it was Kinsey who coined the "missionary position". Anyhow, it's a contemptuous description by the Melanesian natives of a white man's ridicule and clumsiness.
A charming booklet on Melanesian missionaries was also published by the Anglican Church. It appeared one year after Kinsey and two decades after Malinowski, and so it may contain additional information on the subject. Unfortunately we have not been able to consult it.