Evangelicals like to tell us that sex should take place within the confines of marriage, and only for purposes of procreation. But in fact, the situation is much more complex. In his book Law, Sex, and Christian Society in Medieval Europe (1987), James A. Brundage compiled a list of various restrictions on sex from religious sources, and put them together in this handy flowchart (click to embiggen):
(Thanks to for the pointer.)
Epsilon Clue 