Although the Bible does not mention sexual positions, from the 6th to 16th centuries, some Church authorities taught that intercourse should be face-to-face, man-on-top, primarily because they believed that semen flows with gravity, leading to conception. Exceptions were made for couples dealing with illness, obesity, or pregnancy. According to John Bancroft's "Human Sexuality and Its Problems", Thomas Aquinas believed that crimes against nature included intercourse in "unnatural" ways. Protestants did not communicate proper sex positions, and the Catholic Church eventually abandoned its discourse on the topic. Simon Hardy wrote that the missionary position was used to distinguish "bestial and civilized sex."