Over the centuries, vulgar words like fuck and cunt have been included dictionaries, then cast out, then in again, then out, in, out, and so on.
One major problem dictionary editors face in defining sexual terms is deciding how explicit to be. Defining coitus as "an act of sexual intercourse" but leaving sexual intercourse undefined, for example (on the grounds that a reader could figure it out from the definitions of sexual and intercourse), would be a problem, not only because it makes the reader do too much page-flipping but also because the definitions probably still won't be sufficiently clear.
The rest of the article, by Jesse Sheidlower, the editor-at-large of the Oxford English Dictionary, is deliciously vulgar and informative so be wary if you're easily offended and don't like information.