On Mary Douglas from Wikipedia:

Mary Douglas - Wikipedia, the free encyclopedia: "Douglas is best known for her interpretation of the book of Leviticus, and for her role in creating the Cultural Theory of risk.

In Purity and Danger, Douglas first proposed that the kosher laws were not, as many believed, either primitive health regulations or randomly chosen as tests of Jews' commitment to God. Instead, Douglas argued that the laws were about symbolic boundary-maintenance. Prohibited foods were those which did not seem to fall neatly into any category. For example, pigs' place in the natural order was ambiguous because they shared the cloven hoof of the ungulates, but did not chew cud.

Douglas claims that rituals of purity that focus on sexuality are meant to mark the boundaries of the human body, in the same way by which the boundaries of society are marked.

She begins 'Purity and Danger' at stating what she considers obvious, that “ambiguous things can seem very threatening” (xi) and claim that “taboo is a spontaneous device for protecting the distinctive categories of the universe… taboo confronts the ambiguous and shunt it into the category of the sacred”."