Coitus Interruptus: The worst form of birth control only 2nd to the "Pray" method (personally, I think the rhythm method goes under the Pray category). This is a very unreliable method. When they warn about precum containing sperm, they're not kidding. One of my friends got a virgin pregnant because he got some of his precum on her vagina.
He never even penetrated her. True story.
http://www.netdoctor.co.uk/sex_relat...nterruptus.htm
http://www.wikipedia.org/wiki/Coitus_interruptus